Using a new grism at the Keck Interferometer, we obtained spectrally dispersed (R ~ 230) interferometric measurements of the Mira star R Vir. These data show that the measured radius of the emission varies substantially from 2.0-2.4 microns. Simple models can reproduce these wavelength-dependent variations using extended molecular layers, which absorb stellar radiation and re-emit it at longer wavelengths. Because we observe spectral regions with and without substantial molecular opacity, we determine the stellar photospheric radius, uncontaminated by molecular emission. We infer that most of the molecular opacity arises at approximately twice the radius of the stellar photosphere.
We present measurements of the higher-order clustering of red and blue galaxies as a function of scale and luminosity made from the two-degree field galaxy redshift survey (2dFGRS). We use a counts-in-cells analysis to estimate the volume averaged correlation functions, xi_p, as a function of scale up to order p=5, and also the reduced void probability function. Hierarchical amplitudes are constructed using the estimates of the correlation functions: S_p=(xi_p/xi_2)^(p-1). We find that: 1) Red galaxies display stronger clustering than blue galaxies at all orders measured. 2) Red galaxies show values of S_p that are strongly dependent on luminosity, whereas blue galaxies show no segregation in S_p within the errors; this is remarkable given the segregation in the variance. 3) The linear relative bias shows the opposite trend to the hierarchical amplitudes, with little segregation for the red sequence and some segregation for the blue. 4) Faint red galaxies deviate significantly from the "universal" negative binomial reduced void probabilities followed by all other galaxy populations. Our results show that the characteristic colour of a galaxy population reveals a unique signature in its spatial distribution, and we discuss how this can provide new constraints for models of galaxy formation. Such constraints will hopefully further elucidate the physics responsible for shaping the cosmological evolution of galaxies.
A survey for candidate Herbig-Haro objects was performed to search for evidence of star formation in Galactic dark clouds. For this aim true colour images were created from blue, red, and infrared DSS2 plates and inspected. Follow-up I-band, Halpha, and [SII] CCD imaging and long-slit spectroscopy using the Tautenburg 2-m telescope was carried out to verify candidate objects. In the case of LDN 1415, the presence of a Herbig-Haro flow could be revealed which is henceforth named HH 892. In addition, an arcuate nebula was found which is barely seen on the DSS2 infrared plate (epoch 1996) and not detected in archival Kiso Schmidt data (epoch 2001). Thus, this nebula must have brightened by about 3.8mag in recent years.The nebula is associated with IRAS 04376+5413. The 2MASS images show a red counterpart of the IRAS source, designated as L1415-IRS. Its morphology resembles that of a bipolar young object. The luminosity of this source integrated from 0.9micron to 60micron in the low state amounts to 0.13L_sun for an assumed distance of 170pc. Thus it seems to be a young very-low mass star or it might even be of substellar mass. The current brightness increase of the nebula is caused by a FUor- or EXor-like outburst as indicated by the presence of a P Cygni profile of the Halpha line. L1415-IRS is by far the least luminous member of the sparse sample of FUor and EXor objects.
(Abridged) We describe the results of three-dimensional (3D) numerical simulations designed to study turbulent convection in the stellar interiors, and compare them to stellar mixing-length theory (MLT). Simulations in 2D are significantly different from 3D, both in terms of flow morphology and velocity amplitude. Convective mixing regions are better predicted using a [dynamic boundary condition] based on the bulk Richardson number than by purely local, static criteria like Schwarzschild or Ledoux. MLT gives a good description of the velocity scale and temperature gradient for a mixing length of $\sim 1.1 H_p$ for shell convection, however there are other important effects that it does not capture near boundaries. Convective "overshooting" is best described as an elastic response by the convective boundary, rather than ballistic penetration of the stable layers by turbulent eddies. We find that the rate at which material entrainment proceeds at the boundaries is consistent with analogous laboratory experiments as well as simulation and observation of terrestrial atmospheric mixing. In particular, the normalized entrainment rate E=$u_E/\sigma_H$, is well described by a power law dependence on the bulk Richardson number $Ri_B = \Delta b L/\sigma_H^2$ for the conditions studied, $20\lesssim Ri_B \lesssim 420$. We find $E = A Ri_B^{-n}$, with best fit values, $\log A = 0.027 \pm 0.38$, and $n = 1.05 \pm 0.21$. We discuss the applicability of these results to stellar evolution calculations.
In order to analyze the importance of the star formation rate in generating and amplifying magnetic fields in the interstellar medium we obtained deep total power and polarization maps at 4.85 GHz of three late type spiral galaxies NGC4236, NGC4656 and IC2574 using the 100-m Effelsberg radio telescope. This was accompanied by imaging in the Halpha line. We also observed these objects at 1.4 GHz to obtain their integrated fluxes at this frequency and to determine their radio spectra. All galaxies were found to possess weak but detectable total power emission at 4.85 GHz, coincident with regions of recent star formation as traced by bright Halpha regions. The surface brightness of the radio-strongest object of our sample (NGC4656) is comparable to the radio-weakest objects in a sample of more than 50 normally star-forming spiral galaxies for which measurements at 4.8 GHz with the Effelsberg radio telescope are available. The fractional polarization of the 2 galaxies of our sample is less than 2%, significantly lower than for spiral galaxies of intermediate types, suggesting that the magnetic fields are not only weaker, but also less ordered than in spiral galaxies. The radio spectra of galaxies in our small sample are indicative of a substantial fraction of thermal emission, with a higher thermal fraction than in spirals with high star formation rates (SFR), while the nonthermal emission in our sample is relatively weak compared to spiral galaxies. We propose an equipartition model where the nonthermal emission increases ~SFR^(~1.4) and the ratio of nonthermal to thermal emission increases ~SFR^(~0.4). The objects of our sample still follow the radio-FIR correlation of surface brightness of the total emission, extending it towards the lowest values measured so far.
In this paper we compare fully compressible (Meakin & Arnett 2006a,b) and anelastic (Kuhlen, Woosley, & Glatzmaier 2003) simulations of stellar oxygen shell burning. It is found that the two models are in agreement in terms of the velocity scale (v_c ~ 1e7 cm/s) and thermodynamic fluctuation amplitudes (e.g., rho'/<rho> ~ 2e-3) in the convective flow. Large fluctuations (~11%) arise in the compressible model, localized to the convective boundaries, and are due to internal waves excited in stable layers. Fluctuations on the several percent level are also present in the compressible model due to composition inhomogeneities from ongoing entrainment events at the convective boundaries. Comparable fluctuations (with amplitudes greater than ~1%) are absent in the anelastic simulation because they are due to physics not included in that model. We derive an analytic estimate for the expected density fluctuation amplitudes at convective boundaries by assuming that the pressure fluctuations due to internal waves at the boundary, p_w', balance the ram pressure of the convective motions, rho*v_c2. The predicted amplitudes agree well with the simulation data. The good agreement between the anelastic and the compressible solution within the convection zone and the agreement between the stable layer dynamics and analytic solutions to the non-radial wave equation indicate that the compressible hydrodynamic techniques used are robust for the simulated stellar convection model, even at the low Mach n umbers found M~0.01.
The giant flare of December 27, 2004 from SGR 1806-20 represents one of the most extraordinary events captured in over three decades of monitoring the gamma-ray sky. One measure of the intensity of the main peak is its effect on X- and gamma-ray instruments. RHESSI, an instrument designed to study the brightest solar flares, was completely saturated for ~0.5 s following the start of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the time of the giant flare, however, allowed RHESSI a unique view of the giant flare event, including the precursor, the main peak decay, and the pulsed tail. Since RHESSI was saturated during the main peak, we augment these observations with Wind and RHESSI particle detector data in order to reconstruct the main peak as well. Here we present detailed spectral analysis and evolution of the giant flare. We report the novel detection of a relatively soft fast peak just milliseconds before the main peak, whose timescale and sizescale indicate a magnetospheric origin. We present the novel detection of emission extending up to 17 MeV immediately following the main peak, perhaps revealing a highly-extended corona driven by the hyper-Eddington luminosities. The spectral evolution and pulse evolution during the tail are presented, demonstrating significant magnetospheric twist and evolution during this phase. Blackbody radii are derived for every stage of the flare, which show remarkable agreement despite the range of luminosities and temperatures covered. Finally, we place significant upper limits on afterglow emission in the hundreds of seconds following the giant flare.
A long debated issue concerning the nucleosynthesis of neutron-rich elements in Asymptotic Giant Branch (AGB) stars is the identification of the neutron source. We report intermediate-mass (4 to 8 solar masses) AGB stars in our Galaxy that are rubidium-rich owing to overproduction of the long-lived radioactive isotope 87Rb, as predicted theoretically 40 years ago. This represents a direct observational evidence that the 22Ne(alpha,n)25Mg reaction must be the dominant neutron source in these stars. These stars then challenge our understanding of the late stages of the evolution of intermediate-mass stars and would promote a highly variable Rb/Sr environment in the early solar nebula.
We revisit the case of magneto-rotational evolution of neutron stars (NSs) with accounting for changes of the angle \chi between spin and magnetic axes. This element of the evolution of NSs is very important for age estimates and population modeling, but usually it is neglected. In the framework of two models of energy losses we demonstrate that unless NSs are born with the inclination angle \chi_0 very close (within 1 degree) to the position of maximal losses, pulsars with short initial periods quickly reach regions of small energy losses (aligned rotators in the case of magneto-dipole losses and orthogonal rotators -- in the case of current losses) without significant spin-down. This means that either most of known NSs should be born with relatively long periods close to presently observed values, or the initial inclination angles should be large (close to 90) for the magnetodipole model and very small (close to 0) for the current losses model, or that both models cannot be applied for the whole evolutionary track of a typical NS. In particular, magnetar candidates should be born with periods close to the observed ones within the scope of both models of energy losses. We discuss how these considerations can influence population synthesis models and age estimates for different types of isolated NSs. Focusing on the model of current losses we illustrate our conclusion with evolutionary track reconstructions on the P--\sin \chi plane. We conclude, that most probably both existing models of energy losses -- magneto-dipole and longitudinal current -- require serious modifications, on the other hand, estimates obtained under the standard assumption of \sin \chi=1=const does not have solid theoretical ground and should be taken with care.
We consider predictions for structure formation from modifications to general
relativity in which the Einstein-Hilbert action is replaced by a general
function of the Ricci scalar. We work without fixing a gauge, as well as in
explicit popular coordinate choices, and present the framework in a
comprehensive and practical form that can be directly compared to standard
perturbation analyses, appropriate for the modification of existing
cosmological codes.
By considering the full evolution equations, we resolve perceived
instabilities previously suggested, and instead find a suppression of
perturbations. This result presents significant challenges for agreement with
current cosmological structure formation observations.
The findings apply to a broad range of forms of f(R) for which the
modification becomes important at low curvatures, disfavoring them in
comparison with the LCDM scenario. As such, these results provide a powerful
method to rule out a wide class of modified gravity models aimed at providing
an alternative explanation to the dark energy problem.
We have used the Hubble Space Telescope's Advanced Camera for Surveys to measure the mass density function of morphologically selected early-type galaxies in the Gemini Deep Deep Survey fields, over the redshift range 0.9 < z < 1.6. Our imaging data set covers four well-separated sight-lines, and is roughly intermediate (in terms of both depth and area) between the GOODS/GEMS imaging data, and the images obtained in the Hubble Deep Field campaigns. Our images contain 144 galaxies with ultra-deep spectroscopy, and they have been analyzed using a new purpose-written morphological analysis code which improves the reliability of morphological classifications by adopting a 'quasi-petrosian' image thresholding technique. We find that at z = 1 approximately 70% of the stars in massive galaxies reside in early-type systems. This fraction is remarkably similar to that seen in the local Universe. However, we detect very rapid evolution in this fraction over the range 1.0 < z < 1.6, suggesting that in this epoch the strong color-morphology relationship seen in the nearby Universe is beginning to fall into place.
Multi-wavelength B, V, R, I observations of the optical afterglow of GRB 050319 were performed by the 1.05-m telescope at Kiso Observatory and the 1.0-m telescope at Lulin Observatory from 1.31 hours to 9.92 hours after the burst. Our R band lightcurves, combined with other published data, can be described by the smooth broken power-law function, with $\alpha_1$ = -0.84 $\pm$0.02 to $\alpha_2$ = -0.48$\pm$0.03, 0.04 days after the GRB. The optical lightcurves are characterized by shallow decays-- as was also observed in the X-rays-- which may have a similar origin, related to energy injection. However, our observations indicate that there is still a puzzle concerning the chromatic breaks in the R band lightcurve (at 0.04 days) and the X-ray lightcurve (at 0.004 days) that remains to be solved.
Grain alignment theory has reached the stage where quantitative predictions
of the degree of alignment and its variations with optical depth are possible.
With the goal of studying the effect of clumpiness on the sub-millimeter and
far infrared polarization we computed the polarization due to alignment via
radiative torques within clumpy models of cores and molecular clouds. Our
models were based upon a highly inhomogeneous simulation of compressible
magnetohydrodynamic turbulence. The P-I relations for our models reproduce
those seen in observations. We show that the degree of polarization observed is
extremely sensitive to the upper grain size cut-off, and is less sensitive to
changes in the radiative anisotropy. Furthermore, despite a variety of dust
temperatures along a single line of sight through our core and amongst dust
grains of different sizes, the assumption of isothermality amongst the aligned
grains does not introduce a significant error. Our calculations indicate that
sub-mm polarization vectors can be reasonably good tracers for the underlying
magnetic field structure, even for relatively dense clouds (A_V~10 to the cloud
center).
The current predictive power of the grain alignment theory should motivate
future polarization observations using the next generation of multi-wavelength
sub-mm polarimeters such as those proposed for SOFIA.
The detection of a nonthermal excess in the Coma cluster spectrum by two BeppoSAX observations analyzed with the XAS package (Fusco-Femiano et al.) has been disavowed by an analysis (Rossetti & Molendi) performed with a different software package (SAXDAS) for the extraction of the spectrum. To resolve this discrepancy we reanalyze the PDS data considering the same software used by Rossetti & Molendi. A correct selection of the data and the exclusion of contaminating sources in the background determination show that also the SAXDAS analysis reports a nonthermal excess with respect to the thermal emission at about the same confidence level of that obtained with the XAS package (~4.8sigma). Besides, we report the lack of the systematic errors investigated by Rossetti & Molendi and Nevalainen et al. taking into account the whole sample of the PDS observations off the Galactic plane, as already shown in our data analysis of Abell 2256 (Fusco-Femiano, Landi & Orlandini). All this eliminates any ambiguity and confirms the presence of a hard tail in the spectrum of the Coma cluster.
Based on VLT/FORS spectroscopy and HST/ACS imaging, we have constructed a sample of ~200 field spiral galaxies that cover redshifts up to z~1. Such a large data set allows to study the evolution of fundamental galaxy parameters like luminosity, size, mass, mass-to-light ratio etc. as a function of cosmic time and in various mass regimes. Several of our findings - like the time-independent fraction of stellar-to-total mass - are in compliance with a hierarchical structure growth. However, the stellar population properties of intermediate-redshift disks favour a down-sizing scenario in the sense that the average stellar ages in high-mass spirals are older than in low-mass spirals.
We searched for 21-cm absorption associated with the z_abs = 1.3647 absorption system toward PKS 0237-233 using the GMRT. A high quality UVES spectrum shows that C I and C I* are detected at this redshift together with C II*, Mg I, Mg II, Si II, Al II, Fe II and Mn II. The complex profiles, spread over ~300 km/s, are fitted with 21 Voigt profile components. None of these components are detected in 21-cm absorption down to a detection limit of \tau(3\sigma)\le 3x10^{-3} (or N(HI)/T_S <10^{17} cm ^-2 K^-1). We derive log N(HI)<19.30$\pm$0.30 using the Lyman alpha absorption line detected in the IUE spectrum of the quasar. Mg II, Si II and Al II column densities are consistent with near solar metallicity and we measure [O/H]>-0.33. Using photoionization models constrained by the fine-structure excitations of C I and C II, and the 21-cm optical depth, we show that the C I absorption arises predominantly either in WIM or WNM in ionization and thermal equilibrium with the meta-galactic UV background dominated by QSOs and star forming galaxies. The estimated thermal pressure of the gas is of the same order of magnitude over different velocity ranges through the absorption profile (2.6\le log [P/k cm^{-3} K]\le 4.0). The gas-phase metallicity corrected for ionization is Z>0.5 Z_\odot with a signature of Fe co-production elements being under abundant compared to \alpha-process elements by ~0.5 dex. At z>1.9, C I absorption is usually associated with H_2 absorption arising from cold gas in DLAs. This system and the z=2.139 toward Tol 1037-270 are the only two systems known which show that C I absorption can also be detected in warm gas provided the metallicity is high enough. Interestingly, both the systems are part of unusual concentrations of absorption lines.
With the aid of a simple yet robust approach we investigate the influence of dissipationless and dissipative merging on galaxy structure, and the consequent effects on the scaling laws followed by elliptical galaxies. Our results suggest that ellipticals cannot be originated by parabolic merging of low mass spheroids only, even in presence of substantial gas dissipation. However, we also found that scaling laws such as the Faber-Jackson, Kormendy, Fundamental Plane, and the Mbh -sigma relations, when considered over the whole mass range spanned by ellipticals in the local universe, are robust against merging. We conclude that galaxy scaling laws, possibly established at high redshift by the fast collapse in pre-existing dark matter halos of gas rich and clumpy stellar distributions, are compatible with a (small) number of galaxy mergers at lower redshift.
We investigate the composition and shape distribution of silicate dust grains in the interstellar medium. The effect of the amount of magnesium in the silicate lattice is studied. We fit the spectral shape of the interstellar 10 mu extinction feature as observed towards the galactic center. We use very irregularly shaped coated and non-coated porous Gaussian Random Field particles as well as a statistical approach to model shape effects. For the dust materials we use amorphous and crystalline silicates with various composition and SiC. The results of our analysis of the 10 mu feature are used to compute the shape of the 20 mu silicate feature and to compare this with observations. By using realistic particle shapes we are, for the first time, able to derive the magnesium fraction in interstellar silicates. We find that the interstellar silicates are highly magnesium rich (Mg/(Fe+Mg)>0.9) and that the stoichiometry lies between pyroxene and olivine type silicates. This composition is not consistent with that of the glassy material found in GEMS in interplanetary dust particles indicating that these are, in general, not unprocessed remnants from the interstellar medium. Also, we find a significant fraction of SiC (~3%). We discuss the implications of our results for the formation and evolutionary history of cometary and circumstellar dust. We argue that the fact that crystalline silicates in cometary and circumstellar grains are almost purely magnesium silicates is a natural consequence of our findings that the amorphous silicates from which they were formed were already magnesium rich.
We present the CXOCY J220132.8-320144 system, which is composed of an edge-on spiral galaxy at z=0.32 lensing a z=3.9 background quasar. Two images of the quasar are seen. The geometry of the system is favorable to separate the relative mass contribution of the disk and halo in the inner parts of the galaxy. We model the system with one elliptical mass component with the same ellipticity as the light distribution and manage to reproduce the quasar image positions and fluxes. We also model the system with two mass components, disk and halo. Again, we manage to reproduce the quasar image positions and fluxes. However, all models predict at least a third visible image close to the disk that is not seen in our images. We speculate that this is most likely due to extinction by the disk. We also measure the rotational velocity of the galaxy at 2.7 disk scale radius to be v_c=130 +/- 20 km/s from the [OII] emission lines. When adding the rotational velocity constraint to the models, we find that the contribution to the rotational velocity of the disk is likely to be equal to or larger than the contribution of the halo at this radius. The detection of the third image and a more accurate measurement of the rotational velocity would help to set tighter constraints on the mass distribution of this edge-on spiral galaxy.
A variety of pulsar studies, ranging from high precision astrometry to tests for theories of gravity, require high time resolution data. Few such observations at more than two frequencies below 1 GHz are available. Giant Meterwave Radio Telescope (GMRT) has the unique capability to provide such multi-frequency pulsar data at low observation frequencies, but the quality and time resolution of pulsar radio signals is degraded due to dispersion in the inter-stellar medium at these frequencies. Such degradation is usually taken care of by employing specialized digital hardware, which implement coherent dedispersion algorithm. In recent years, a new alternative is provided by the availability of cheap computer hardware. In this approach, the required signal processing is implemented in software using commercially off-the-shelf available computing hardware. This makes such a receiver flexible and upgradeable unlike a hardware implementation. The salient features and the modes of operation of a high time resolution pulsar instrument for GMRT based on this approach is described in this paper. The capability of the instrument is demonstrated by illustrations of test observations. We have obtained the average profile of PSR B1937+21 at 235 MHz for the first time and this profile indicates a scattering timescale of about 300 us. Lastly, the possible future extensions of this concept are discussed.
We have observed again two stars inside the ZZ Ceti instability strip that were previously classified as not-observed-to-vary (NOV) by Mukadam et al. (2004) and found them to be low-amplitude variables. Some evidence points to a pure ZZ Ceti instability strip; other evidence contests it. The two stars previously classified as NOV have Sloan Digital Sky Survey (SDSS) spectroscopic effective temperatures that place them inside the ZZ Ceti instability strip, and they were "contaminating" the strip as constant stars, which could indicate that the instability strip was no longer a simple evolutionary stage. A pure instability strip indicates that pulsation is a normal phase which all DAs must go through. We used effective temperatures derived from SDSS optical spectra by comparing them with model atmospheres to look for pulsators through time-resolved photometry and stars previously classified as NOV. Our new results indicate, but do not prove, a pure instability strip, because there are still other NOV stars that need to be observed again. Additionally, we have discovered five other ZZ Ceti stars based on their effective temperatures.
Recently, with the ``bounching photon'' treatment, the gauge invariance of the response of an interferometer to scalar gravitational waves (SGWs) has been demonstred in its full frequency dependence in three different gauges well known in literature, while in previous works it was been shown only in the low frequencies approximation. In this paper the analysis of the response function for SGWs is generalized in its full angular dependence and directly in the gauge of the local observer, which is the gauge of a laboratory enviroment on Earth. The result is used for anlyzing the cross - correlation between the two LIGO interferometers in their advanced configuration for a potential detection of a stochastic bacground of SGWs. An inferior limit for the integration time of a potential detection is released.
Gravitational waves emitted by chirping supermassive black hole binaries could in principle be used to obtain very accurate distance determinations. Provided they have an electromagnetic counterpart from which the redshift can be determined, these standard sirens could be used to build a high redshift Hubble diagram. Errors in the distance measurements will most likely be dominated by gravitational lensing. We show that the (de)magnification due to inhomogeneous foreground matter will increase the scatter in the measured distances by a factor ~10. We propose to use optical and IR data of the foreground galaxies to minimize the degradation from weak lensing. We find that the net effect of correcting the estimated distances for lensing is comparable to increasing the sample size by a factor of three when using the data to constrain cosmological parameters.
The flux upper limits of the diffuse gamma rays, from the inner and outer Galactic planes, are revised by factors of 4.0$\sim$3.7 for mode energies 3$\sim$10 TeV, respectively, by using the simulation results of the effective area ratios for gamma-ray induced showers and cosmic-ray induced ones in the Tibet air shower array. In our previous work, (Amenomori et al., ApJ, 580, 887, 2002) the flux upper limits were deduced only from the flux ratio of air showers generated by gamma rays versus cosmic rays. The details of the simulation are given in the paper (Amenomori et al., Advances in Space Research, 37, 1932, 2006). The present result using the same data as in ApJ suggests that the spectral index of source electrons is steeper than 2.2 and 2.1 for the inner and outer Galactic planes, respectively.
We have carried out a direct and differential imaging search for sub-stellar companions to eps Indi A using the adaptive optics system NACO at the ESO VLT. The observations were carried out in September 2004 with NACO/SDI as well as with NACO's S27 camera in the H and Ks filters. The SDI data cover an area of \~2.8" around eps Indi A. No detection was achieved in the inner neighbourhood down to 53 Mj (5 sigma confidence level) at a separation > 0.4" (1.45 AU) and down to 21 Mj for separations > 1.3" (4.7 AU). To cover a wider field of view, observations with the S27 camera and a coronagraphic mask were obtained. We detected a faint source at a separation of (7.3 +/- 0.1)" and a position angle of (302.9 +/- 0.8) degree. The photometry for the candidate companion yields m(H)=(16.45 +/- 0.04)mag and m(Ks) = (15.41 +/- 0.06)mag, respectively. Those magnitudes and the resulting color (H-Ks) = (1.04 +/- 0.07)mag fit best to a spectral type of L5 - L9.5 if it is bound. Observations done with HST/NICMOS by M. Endl have shown the source to be a background object.
In this article I will briely review the current status of radio observations of nearby supernovae and their remnants. This review will initially address observations of the radio light curves of nearby core-collapse supernovae, followed by a more detailed summary of recent Very Long Baseline Interferometric observations of the expansion of nearby supernovae and their remnants. These later sections will concentrate on a few sources, namely those found in M82, SN1993J, and the recent supernovae SN2004et. In addition I will discuss the many radio detections of supernovae found in the highly obscured centres of starburst galaxies, such as M82 and Arp220, where no optical detections are possible.
We study the temporal evolution of coronal loops using data from the Solar X-ray Imager (SXI) on board of GOES-12. This instrument allows us to follow in detail the full lifetime of coronal loops. The observed light curves suggest three somewhat distinct evolutionary phases: rise, main, and decay. The durations and characteristic timescales of these phases are much longer than a cooling time and indicate that the loop-averaged heating rate increases slowly, reaches a maintenance level, and then decreases slowly. This suggests that a single heating mechanism operates for the entire lifetime of the loop. For monolithic loops, the loop-averaged heating rate is the intrinsic energy release rate of the heating mechanism. For loops that are bundles of impulsively heated strands, it is an indication of the frequency of occurrence of individual heating events, or nanoflares. We show that the timescale of the loop-averaged heating rate is proportional to the timescale of the observed intensity variation. The ratios of the radiative to conductive cooling times in the loops are somewhat less than 1, putting them intermediate between the values measured previously for hotter and cooler loops. Our results provide further support for the existence of a trend suggesting that all loops are heated by the same mechanism, or that different mechanisms have fundamental similarities (e.g., are all impulsive or are all steady with similar rates of heating).
Wide-field survey instruments are used to efficiently observe large regions of the sky. To achieve the necessary field of view, and to provide a higher signal-to-noise ratio for faint sources, many modern instruments are undersampled. However, precision photometry with undersampled imagers requires a detailed understanding of the sensitivity variations on a scale much smaller than a pixel. To address this, a near-infrared spot projection system has been developed to precisely characterize near-infrared focal plane arrays and to study the effect of sub-pixel non uniformity on precision photometry. Measurements of large format near-infrared detectors demonstrate the power of this system for understanding sub-pixel response.
The submillimeter CO lines are formed in warm and dense molecular gas and are therefore sensitive to the physical conditions whereas the CO (J=1-0) line is a tracer of the total molecular gas mass. In order to be able to compare the properties of molecular gas in nearby and distant galaxies, we have observed C and CO submillimeter lines (including the 12CO(6-5) and 12CO(7-6) lines) in a sample of nearby galaxies using the CSO. We have obtained a complete view of the CO cooling curve (also called CO spectral energy distribution) by combining the submillimeter CSO data with previous observations found in the literature. We made use of Large Velocity Gradient (LVG) models to analyse the observed CO cooling curve, predict CO line intensities from J=1-0 to J=15-14 in the studied galaxies, and derive the physical properties of the warm and dense molecular gas : the kinetic temperature (TK); the gas density (n(H2)); the N(12CO)/$\Delta v$. The predictions for the line intensities and for the total CO cooling power, obtained from LVG modelling have been compared with predictions from Photo Dissociation Regions (PDR) models. We show how the CO SED varies according to the galaxy star forming activity. For active nuclei, the peak is located near the 12CO(6-5) or 12CO(7-6) rotational lines, while, for normal nuclei, most of the energy is carried by the 12CO(4-3) and 12CO(5-4) lines. Whatever the spectral type of the nucleus, the observed C cooling rate is lower than the observed CO cooling rate (by a factor of around 4). The CO cooling curve of nearby starburst galaxies (e.g. NGC 253) has a quite similar shape to the CO cooling curve of distant galaxies. Therefore, the CO cooling curves are useful diagnostics for the star forming activity in distant objects.
We report the results of a series of RXTE and ATCA observations of the recently-discovered X-ray transient IGR J17497-2821. Our 3-200 keV PCA+HEXTE spectral analysis shows very little variations over a period of ~10 days around the maximum of the outburst. IGR J17497-2821 is found in a typical Low Hard State (LHS) of X-ray binaries (XRB), well represented by an absorbed Comptonized spectrum with an iron edge at about 7 keV. The high value of the absorption (~4e22 cm-2) suggests that the source is located at a large distance, either close to the Galactic center or beyond. The timing analysis shows no particular features, while the shape of the power density spectra is also typical of LHS of XRBs, with ~36% RMS variability. No radio counterpart is found down to a limit of 0.21 mJy at 4.80 GHz and 8.64 GHz. We discuss the properties of IGR J17497-2821 and by comparing them with those of other sources, we suggest that it is a black hole candidate.
It is shown that, for Broad Line Radio Galaxies the strength of the non-thermal beamed radiation, when present, is always smaller than the accretion flow by a factor < 0.7 in the 2-10 keV band. The result has been obtained using the procedure adopted for disentangling the Flat Spectrum Radio Quasar 3C 273 (Grandi & Palumbo 2004). Although this implies a significantly smaller non-thermal flux in Radio Galaxies when compared to Blazars, the jet component, if present, could be important at very high energies and thus easily detectable with GLAST.
IGR J16318-4848 is the prototype and one of the more extreme examples of the new class of highly obscured Galactic X-ray sources discovered by INTEGRAL. A monitoring campaign on this source has been carried out by XMM-Newton and INTEGRAL, consisting in three simultaneous observations performed in February, March and August 2004. The long-term variability of the Compton-thick absorption and emission line complexes will be used to probe the properties of the circumstellar matter. A detailed timing and spectral analysis of the three observations is performed, along with the reanalysis of the XMM-Newton observation performed in February 2003. The results are compared with predictions from numerical radiative transfer simulations to derive the parameters of the circumstellar matter. Despite the large flux dynamic range observed (almost a factor 3 between observations performed a few months apart), the source remained bright (suggesting it is a persistent source) and Compton-thick (NH >1.2x10^24 cm-2). Large Equivalent Width (EW) emission lines from Fe Kalpha, Fe Kbeta and Ni Kalpha were present in all spectra. The addition of a Fe Kalpha Compton Shoulder improves the fits, especially in the 2004 observations. Sporadic occurrences of rapid X-ray flux risings were observed in three of the four observations. The Fe Kalpha light curve followed the continuum almost instantaneously, suggesting that the emission lines are produced by illumination of small-scale optically-thick matter around the high-energy continuum source. Using the iron line EW and Compton Shoulder as diagnostic of the geometry of the matter, we suggest that the obscuring matter is in a flattened configuration seen almost edge-on.
We report on a Chandra grating observation of the recently discovered hard X-ray transient IGR J17497-2821. The observation took place about two weeks after the source discovery at a flux level of about 20 mCrab in the 0.8-8 keV range. We extracted the most precise X-ray position of IGR J17497-2821, RA=17 49 38.037, DEC= -28 21 17.37 (90% uncertainty of 0.6"). We also report on optical and near infra-red photometric follow-up observations based on this position. With the multi-wavelength information at hand, we discuss the possible nature of the source proposing that IGR J17497-2821 is a low-mass X-ray binary, most likely hosting a black hole, with a red giant K-type companion.
Compact remnants -- stellar mass black holes and neutron stars formed in the inner few parsec of galactic centres are predicted to sink into the central parsec due to dynamical friction on low mass stars, forming a high concentration cusp (Morris 1993). Same physical region may also contain very high density molecular clouds and accretion discs that are needed to fuel SMBH activity. Here we estimate gas capture rates onto the cusp of stellar remnants, and the resulting X-ray luminosity, as a function of the accretion disc mass. At low disc masses, most compact objects are too dim to be observable, whereas in the high disc case most of them are accreting at their Eddington rates. We find that for low accretion disc masses, compact remnant cusps may be more luminous than the central SMBHs. This ``diffuse'' emission may be of importance for local moderately bright AGN, especially Low Luminosity AGN. We also briefly discuss how this expected emission can be used to put constraints on the black hole cusp near our Galactic Centre.
The Spitzer Space Telescope and in particular its InfraRed Array Camera (IRAC) is an ideal facility to study the distribution of AGB stars in our own and other galaxies because of its efficiency in surveying vast areas of the sky and its ability to detect sources with infrared excess. The IRAC colors of AGB stars, however, are not well known because cool stars have numerous molecular absorption features in the spectral region covered by the IRAC photometric system. The presence and strength of these features depends on the chemistry of the stellar atmosphere and the mass loss rate and can change with time due to the star's variability. To characterize the IRAC colors of AGB stars, we are carrying out a Spitzer Guaranteed Time Observation program to observe a sample of AGB stars with IRAC. The results will be made available to the community in the form of template magnitudes and colors for each target with the goal of aiding the identification of AGB stars in already available and future IRAC surveys. We present here the first results of this project.
Straight-forward models of X-ray reflection in the inner region of accretion discs predict that primary X-ray flux and the flux reflected off the surface of the disc should vary together, albeit a short light travel time delay. Most of the observations, however, show that the X-ray flux can vary while the reflected features remain constant. Here we propose a simple explanation to this. In all likelihood, the emission of a moderately optically thick magnetic flare atop an accretion disc is anisotropic. A constant energy release rate in a flare will appear to produce a variable X-ray flux as the flare rotates with the accretion disc anchoring the magnetic tube. The reflector, on the other hand, receives a constant X-ray flux from the flare. Since the reflected emission is azimuthally symmetric, the observer will see a roughly constant reflected flux (neglecting relativistic effects). The model does not produce quasi-periodic oscillations (QPO) if magnetic flux tubes are sheared out faster than they complete one orbit.
We present X-ray observations of the Narrow-Line Seyfert 1 galaxy PKS 0558-504 obtained with RXTE during a 1-year monitoring campaign. This source, which is one of the very few radio-loud NLS1 galaxies, shows strong X-ray flux variability on time scales of weeks-months accompanied by spectral variability. The main goal of this study is to investigate the spectral variability with model-independent methods and time-resolved spectroscopy in order to shed light on the origin of the X-rays. The main results can be summarized as follows: 1) The flux typically changes by a factor of 1.5-2 on time scales of 10-30 days, with few extreme events where the flux increases by a factor of ~4 in 3 days. 2) We do not observe any large amplitude, flux related spectral variations. During the flux variations, the spectrum varies mainly in normalization and not in shape. We do observe some small amplitude spectral variations, which do not correlate with flux, although there is a hint of spectral hardening as the source brightens. 3) There is no evidence for reprocessing features such as the Fe Kalpha line or a Compton hump. We argue that PKS 0558-504 is a peculiar object that appears to be different from most of the radio-quiet and radio-loud AGN. If a jet is responsible for the bulk of the X-rays, it must operate in an unusual way. If instead a corona is responsible for the X-rays, the system might be a large-scale analog of the Galactic black holes in the transient intermediate state.
This is the first of two papers reporting observations and analysis of the unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line quasar PHL 1811, focusing on the X-ray properties and the spectral energy distribution. Two Chandra observations reveal a weak X-ray source with a steep spectrum. Variability by a factor of 4 between the two observations separated by 12 days suggest that the X-rays are not scattered emission. The XMM-Newton spectra are modelled in the 0.3--5 keV band by a steep power law with \Gamma = 2.3\pm 0.1, and the upper limit on intrinsic absorption is 8.7 x 10^{20} cm^{-2}. The spectral slopes are consistent with power law indices commonly observed in NLS1s, and it appears that we observe the central engine X-rays directly. Including two recent Swift ToO snapshots, a factor of ~5 variability was observed among the five X-ray observations reported here. In contrast, the UV photometry obtained by the XMM-Newton OM and Swift UVOT, and the HST spectrum reveal no significant UV variability. The \alpha_{ox} inferred from the Chandra and contemporaneous HST spectrum is -2.3 \pm 0.1, significantly steeper than observed from other quasars of the same optical luminosity. The steep, canonical X-ray spectra, lack of absorption, and significant X-ray variability lead us to conclude that PHL 1811 is intrinsically X-ray weak. We also discuss an accretion disk model, and the host galaxy of PHL 1811.
In this review, I give a brief summary of galaxy evolution processes in hierarchical cosmologies and of their relative importance at different masses, times, and environments. I remind the reader of the processes that are commonly included in modern semi-analytic models of galaxy formation, and I comment on recent results and open issues.
We have developed a systematic procedure to study the disks in Algol-type
binaries using spectroscopic analysis, synthetic spectra, and tomography. We
analyzed 119 H-alpha spectra of TT Hya, an Algol-type eclipsing interacting
binary, collected from 1985-2001. The new radial velocities enabled us to
derive reliable orbital elements, including a small non-zero eccentricity, and
to improve the accuracy of the absolute dimensions of the system. High
resolution IUE spectra were also analyzed to study the formation of the
ultraviolet lines and continuum. Synthetic spectra of the iron curtain using
our new shellspec program enabled us to derive a characteristic disk
temperature of 7000K. We have demonstrated that the UV emission lines seen
during total primary eclipse cannot originate from the accretion disk, but most
likely arise from a hotter disk-stream interaction region.
The synthetic spectra of the stars, disk, and stream allowed us to derive a
lower limit to the mass transfer rate of 2e-10 solar masses per year. Doppler
tomography of the observed H-alpha profiles revealed a distinct accretion disk.
The difference spectra produced by subtracting the synthetic spectra of the
stars resulted in an image of the disk, which virtually disappeared once the
composite synthetic spectra of the stars and disk were used to calculate the
difference spectra. An intensity enhancement of the resulting tomogram revealed
images of the gas stream and an emission arc. We successfully modeled the gas
stream using shellspec and associated the emission arc with an asymmetry in the
accretion disk.
Precise measurements of the anisotropies in the cosmic microwave background enable us to do an accurate study on the form of the primordial power spectrum for a given set of cosmological parameters. In a previous paper (Shafieloo and Souradeep 2004), we implemented an improved (error sensitive) Richardson-Lucy deconvolution algorithm on the measured angular power spectrum from the first year of WMAP data to determine the primordial power spectrum assuming a concordance cosmological model. This recovered spectrum has a likelihood far better than a scale invariant, or, `best fit' scale free spectra (\Delta ln L = 25 w.r.t. Harrison Zeldovich, and, \Delta ln L = 11 w.r.t. power law with n_s=0.95). In this paper we use Discrete Wavelet Transform (DWT) to decompose the local features of the recovered spectrum individually to study their effect and significance on the recovered angular power spectrum and hence the likelihood. We show that besides the infra-red cut off at the horizon scale, the associated features of the primordial power spectrum around the horizon have a significant effect on improving the likelihood. The strong features are localised at the horizon scale.
We study the possibility of Late Forming Dark Matter (LFDM), where a scalar field, previously trapped in a metastable state by thermal or finite density effects, begins to oscillate near the era matter-radiation equality about its true minimum. Such a theory is motivated generally if the dark energy is of a similar form, but has not yet made the transition to dark matter, and, in particular, arises automatically in recently considered theories of neutrino dark energy. If such a field comprises the present dark matter, the matter power spectrum typically shows a sharp break at small, presently nonlinear scales, below which power is highly suppressed and previously contained acoustic oscillations. If, instead, such a field forms a subdominant component of the total dark matter, such acoustic oscillations may imprint themselves in the linear regime.
We present a preliminary analysis of an extensive set of optical observations of the Type Ia SN 2005hk. We show that the evolution of SN 2005hk closely follows that of the peculiar SN 2002cx. SN 2005hk is more luminous than SN 2002cx, while still under-luminous compared to normal Type Ia supernovae. The spectrum at 9 days before maximum is dominated by conspicuous Fe III and Ni III lines, and the Si II 6355 line is also clearly visible. All these features have low velocity (~6000 km/s). The near maximum spectra show lines of Si II, S II, Fe II, Fe III, as well as other intermediate mass and iron group elements. Analysis with the code for synthetic spectra SYNOW indicates that all these spectral lines have similar velocities.
We present new HI observations of the nearby dwarf galaxy NGC 3741. This
galaxy has an extremely extended HI disk, which allows us to trace the rotation
curve out to unprecedented distances in terms of the optical disk: we reach 42
B-band exponential scale lengths or about 7 kpc. The HI disk is strongly
warped, but the warp is very symmetric. The distribution and kinematics are
accurately derived by building model data cubes, which closely reproduce the
observations. In order to account for the observed features in the data cube,
radial motions of the order of 5-13 km/s are needed. They are consistent with
an inner bar of several hundreds of pc and accretion of material in the outer
regions.
The observed rotation curve was decomposed into its stellar, gaseous and dark
components. The Burkert dark halo (with a central constant density core)
provides very good fits. The dark halo density distribution predicted by the
LambdaCDM theory fails to fit the data, unless NGC 3741 is a 2.5-sigma
exception to the predicted relation between concentration parameter and virial
mass and has at the same time a high value of the virial mass (though poorly
constrained) of 10$^{11}$ solar masses. Noticeably, MOND seems to be consistent
with the observed rotation curve. Scaling up the contribution of the gaseous
disk also gives a good fit.
We present four spectra of the Type Ia supernova (SN Ia) 2006D extending from -7 to +13 days with respect to B-band maximum. The spectra include the strongest signature of unburned material at photospheric velocities observed in a SN Ia to date. The earliest spectrum exhibits C II absorption features below 14,000 km/s, including a distinctive C II \lambda 6580 absorption feature. The carbon signatures dissipate as the SN approaches peak brightness. In addition to discussing implications of photospheric-velocity carbon for white dwarf explosion models, we outline some factors that may influence the frequency of its detection before and around peak brightness. Two effects are explored in this regard, including depopulation of the C II optical levels by non-LTE effects, and line-of-sight effects resulting from a clumpy distribution of unburned material with low volume-filling factor.
A database with a high density of accurate distances is used to investigate the contributions to the motion of our Galaxy. It is found that the motion of the Local Group separates remarkably cleanly into 3 components: a large-scale attraction toward structure in the `Great Attractor' sector, a mid-scale attraction toward the Virgo Cluster, and a local-scale `repulsion' from the Local Void. These 3 components cause motions of comparable amplitudes and, conveniently, are directed almost orthogonal to one another.
General relativistic deflection of light by mass, dipole, and quadrupole moments of gravitational field of a moving massive planet in the Solar system is derived. All terms of order 1 microarcsecond are taken into account, parametrized, and classified in accordance with their physical origin. We calculate the instantaneous patterns of the light-ray deflections caused by the monopole, the dipole and the quadrupole moments, and derive equations describing apparent motion of the deflected position of the star in the sky plane as the impact parameter of the light ray with respect to the planet changes due to its orbital motion. The present paper gives the physical interpretation of the observed light-ray deflections and discusses the observational capabilities of the near-future optical (SIM) and radio (SKA) interferometers for detecting the Doppler modulation of the radial deflection, and the dipolar and quadrupolar light-ray bendings by the Jupiter and the Saturn.
Angular momentum transport in protostellar discs can take place either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field that threads the disc. Using semi-analytic and numerical results, we construct a model of steady-state discs that includes vertical transport by a centrifugally driven wind as well as MRI-induced turbulence. We present approximate criteria for the occurrence of either one of these mechanisms in an ambipolar diffusion-dominated disc. We derive ``strong field'' solutions in which the angular momentum transport is purely vertical and ``weak field'' solutions that are the stratified-disc analogues of the previously studied MRI channel modes; the latter are transformed into accretion solutions with predominantly radial angular-momentum transport when we implement a turbulent-stress prescription based on published results of numerical simulations. We also analyze ``intermediate field strength'' solutions in which both modes of transport operate at the same radial location; we conclude, however, that significant spatial overlap of these two mechanisms is unlikely to occur in practice. To further advance this study, we have developed a general scheme that incorporates also the Hall and Ohm conductivity regimes in discs with a realistic ionization structure.
We study collective wind configurations produced by a number of massive stars, and obtain densities and expansion velocities of the stellar wind gas that is to be target, in this model, of hadronic interactions. We study the expected $\gamma$-ray emission from these regions, considering in an approximate way the effect of cosmic ray modulation. We compute secondary particle production (electrons from knock-on interactions and electrons and positrons from charged pion decay), and solve the loss equation with ionization, synchrotron, bremsstrahlung, inverse Compton, and expansion losses. We provide examples where configurations can produce sources for GLAST satellite, and the MAGIC, HESS, or VERITAS telescopes in non-uniform ways, i.e., with or without the corresponding counterparts. We show that in all cases we studied no EGRET source is expected.
We investigate the dependences of galaxy star-formation history and galaxy morphology on environment, using color and H-alpha equivalent width as star-formation history indicators, using concentration and central surface brightness as morphological indicators, and using clustocentric distance as an environment indicator. Clustocentric distance has the virtue that it can be measured with very high precision over a large dynamic range. We find the following asymmetry between morphological and star-formation history parameters: star-formation history parameters relate directly to the clustocentric distance while morphological parameters relate to the clustocentric distance only indirectly through their relationships with star-formation history. This asymmetry has important implications for the role that environment plays in shaping galaxy properties and it places strong constraints on theoretical models of galaxy formation. Current semi-analytic models do not reproduce this effect.
The COMPTEL unidentified source GRO J 1411-64 was observed by INTEGRAL and
XMM-Newton in 2005. The Circinus Galaxy is the only source detected within the
4$\sigma$ location error of GRO J1411-64, but in here excluded as the possible
counterpart. At soft X-rays, 22 reliable and statistically significant sources
(likelihood $> 10$) were extracted and analyzed from XMM-Newton data. Only one
of these sources, XMMU J141255.6 -635932, is spectrally compatible with GRO
J1411-64 although the fact the soft X-ray observations do not cover the full
extent of the COMPTEL source position uncertainty make an association hard to
quantify and thus risky. At the best location of the source, detections at hard
X-rays show only upper limits, which, together with MeV results obtained by
COMPTEL suggest the existence of a peak in power output located somewhere
between 300-700 keV for the so-called low state. Such a spectrum resembles
those in blazars or microquasars, and might suggest at work by a similar
scenario.
However, an analysis using a microquasar model consisting on a magnetized
conical jet filled with relativistic electrons, shows that it is hard to comply
with all observational constrains. This fact and the non-detection at hard
X-rays introduce an a-posteriori question mark upon the physical reality of
this source, what is discussed here.
We present a low-resolution spectroscopic survey of the Orion nebula which data we release for public use. In this article, we intend to show the possible applications of this dataset analyzing some of the main properties of the nebula. We perform an integral field spectroscopy mosaic of an area of ~5' X 6' centered on the Trapezium region of the nebula, including the ionization front to the south-east. The analysis of the line fluxes and line ratios of both the individual and integrated spectra allowed us to determine the main characteristics of the ionization throughtout the nebula.The final dataset comprises 8182 individual spectra, which sample each one a circular area of ~2.7" diameter. The data can be downloaded as a single row-stacked spectra fits file plus a position table or as an interpolated datacube with a final sampling of 1.5"/pixel. The integrated spectrum across the field-of-view was used to obtain the main integrated properties of the nebula, including the electron density and temperature, the dust extinction, the Halpha integrated flux (after correcting for dust reddening), and the main diagnostic line ratios. The individual spectra were used to obtain line intensity maps of the different detected lines. These maps were used to study the distribution of the ionized hydrogen, the dust extinction, the electron density and temperature, and the helium and oxygen abundance...
The Gamma-ray Large Area Space Telescope (GLAST) is a next generation high energy gamma-ray observatory due for launch in Fall 2007. The primary instrument is the Large Area Telescope (LAT), which will measure gamma-ray flux and spectra from 20 MeV to > 300 GeV and is a successor to the highly successful EGRET experiment on CGRO. The LAT will have better angular resolution, greater effective area, wider field of view and broader energy coverage than any previous experiment in this energy range. An overview of the LAT instrument design and construction is presented which includes performance estimates with particular emphasis on how these apply to strudies of microquasars. The nature and quality of the data that will be provided by the LAT is described with results from recent detailed simulations that illustrate the potential of the LAT to observe gamma ray variability and spectra.
Current experiments aimed at measuring the UHE (E>100 PeV) neutrino flux via detection of coherent Cherenkov radiation are summarized.
We report a dual-band observation at 223 and 654 GHz (460 micron) toward an ultracompact (UC) HII region, G240.31+0.07, with the Submillimeter Array. With a beam size of 1"5 X 0"8, the dust continuum emission is resolved into two clumps, with clump A coincident well with an H2O maser and the UC HII region. The newly discovered clump, B, about 1"3 (~8.3 X 10^3 AU) to the southwest of clump A, is also associated with H2O masers and may be a more recent star-forming site. The continuum flux densities imply an opacity spectral index of \beta = 1.5 +- 0.3, suggestive of a value lower than the canonical 2.0 found in the interstellar medium and in cold, massive cores. The presence of hot (~100 K) molecular gas is derived by the brightness ratio of two H2CO lines in the 223 GHz band. A radial velocity difference of 2.5 +- 0.4 km/s is found between the two clumps in C18O (6-5) emission. The total (nebular and stellar) mass of roughly 58 Msun in the central region is close to, but not by far larger than, the minimum mass required for the two clumps to be gravitationally bound for binary rotation. Our continuum data do not suggest a large amount of matter associated with the H2 knots that were previously proposed to arise from a massive disk or envelope.
The Optical Monitor (OM) on-board XMM-Newton obtained optical/ultraviolet data for the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST), simultaneously with the X-ray detectors. With the XEST OM data, we aim to study the optical and ultraviolet properties of TMC members, and to do correlative studies between the X-ray and OM light curves. In particular, we aim to determine whether accretion plays a significant role in the optical/ultraviolet and X-ray emissions. The Neupert effect in stellar flares is also investigated. Coordinates, average count rates and magnitudes were extracted from OM images, together with light curves with low time resolution (a few kiloseconds). For a few sources, OM FAST mode data were also available, and we extracted OM light curves with high time resolution. The OM data were correlated with Two Micron All Sky Survey (2MASS) data and with the XEST catalogue in the X-rays. The XEST OM catalogue contains 2,148 entries of which 1,893 have 2MASS counterparts. However, only 98 entries have X-ray counterparts, of which 51 of them are known TMC members and 12 additional are TMC candidates. The OM data indicate that accreting stars are statistically brighter in the U band than non-accreting stars after correction for extinction, and have U-band excesses, most likely due to accretion. The OM emission of accreting stars is variable, probably due to accretion spots, but it does not correlate with the X-ray light curve, suggesting that accretion does not contribute significantly to the X-ray emission of most accreting stars. In some cases, flares were detected in both X-ray and OM light curves and followed a Neupert effect pattern, in which the optical/ultraviolet emission precedes the X-ray emission of a flare, whereas the X-ray flux is proportional to the integral of the optical flux.
Magnetars are a sizable subclass of the neutron star census. Their very high magnetic field strengths are thought to be a consequence of rapid (millisecond) rotation at birth in a successful core-collapse supernova. In their first tens of seconds of existence, magnetars transition from hot, extended ``proto-''magnetars to the cooled and magnetically-dominated objects we identify $\sim10^4$ years later as Soft Gamma-ray Repeaters (SGRs) and Anamolous X-ray Pulsars (AXPs). Millisecond proto-magnetar winds during this cooling phase likewise transition from non-relativistic and thermally-driven to magneto-centrifugally-driven, and finally to relativistic and Poynting-flux dominated. Here we review the basic considerations associated with that transition. In particular, we discuss the spindown of millisecond proto-magnetars throughout the Kelvin-Helmholtz cooling epoch. Because of their large reservoir of rotational energy, their association with supernovae, and the fact that their winds are expected to become highly relativistic in the seconds after their birth, proto-magnetars have been suggested as the central engine of long-duration gamma ray bursts. We discuss some of the issues and outstanding questions in assessing them as such.
We report optical extinction properties of dust for a sample of 26 early-type galaxies based on the analysis of their multicolour CCD observations. The wavelength dependence of dust extinction for these galaxies is determined and the extinction curves are found to run parallel to the Galactic extinction curve, which implies that the properties of dust in the extragalactic environment are quite similar to those of the Milky Way. For the sample galaxies, value of the parameter $R_V$, the ratio of total extinction in $V$ band to selective extinction in $B$ & $V$ bands, lies in the range 2.03 - 3.46 with an average of 3.02, compared to its canonical value of 3.1 for the Milky Way. A dependence of $R_V$ on dust morphology of the host galaxy is also noticed in the sense that galaxies with a well defined dust lane show tendency to have smaller $R_V$ values compared to the galaxies with disturbed dust morphology. The dust content of these galaxies estimated using total optical extinction is found to lie in the range $10^4$ to $10^6 \rm M_{\sun}$, an order of magnitude smaller than those derived from IRAS flux densities, indicating that a significant fraction of dust intermixed with stars remains undetected by the optical method. We examine the relationship between dust mass derived from IRAS flux and the X-ray luminosity of the host galaxies.The issue of the origin of dust in early-type galaxies is also discussed.
We present initial results from ``Via Lactea'', the highest resolution simulation to date of Galactic CDM substructure. It follows the formation of a Milky Way-size halo with Mvir=1.8x10^12 Msun in a WMAP 3-year cosmology, using 234 million particles. Over 10,000 subhalos can be identified at z=0: Their cumulative mass function is well-fit by N(>Msub)= 0.0064 (Msub/Mvir)^(-1) down to Msun=4x10^6 Msun. The total mass fraction in subhalos is 5.3%, while the fraction of surface mass density in substructure within a projected distance of 10 kpc from the halo center is 0.3%. Because of the significant contribution from the smallest resolved subhalos, these fractions have not converged yet. Sub-substructure is apparent in all the larger satellites, and a few dark matter lumps are resolved even in the solar vicinity. The number of dark satellites with peak circular velocities above 10 km/s (5 km/s) is 124 (812): of these, 5 (26) are found within 0.1 Rvir, a region that appeared practically smooth in previous simulations. The neutralino self-annihilation gamma-ray emission from dark matter clumps is approximately constant per subhalo mass decade. Therefore, while in our run the contribution of substructure to the gamma-ray luminosity of the Galactic halo amounts to only 40% of the total spherically-averaged smooth signal, we expect this fraction to grow significantly as resolution is increased further. An all-sky map of the expected annihilation gamma-ray flux reaching a fiducial observer at 8 kpc from the Galactic center shows that at the current resolution a small number of subhalos start to be bright enough to be visible against the background from the smooth density field surrounding the observer.
We present the design of an instrument capable of measuring the high energy ($>$60 MeV) muon-induced neutron flux deep underground. The instrument is based on applying the Gd-loaded liquid-scintillator technique to measure the rate of high-energy neutrons underground based on the neutron multiplicity induced in a Pb target. We present design studies based on Monte Carlo simulations that show that an apparatus consisting of a Pb target of 200 cm by 200 cm area by 60 cm thickness covered by a 60 cm thick Gd-loaded liquid scintillator (0.5% Gd content) detector could measure, at a depth of 2000 meters of water equivalent, a rate of $70\pm12$ (stat) events/year. Based on these studies, we also discuss the benefits of using a neutron multiplicity meter as a component of active shielding in such experiments.
We present low-resolution spectroscopy of 120 red giants in the Galactic satellite dwarf spheroidal (dSph) Leo I, obtained with the GeminiN-GMOS and Keck-DEIMOS spectrographs. We find stars with velocities consistent with membership of Leo I out to 1.3 King tidal radii. By measuring accurate radial velocities with a median measurement error of 4.6 km/s we find a mean systemic velocity of 284.2 km/s with a global velocity dispersion of 9.9 km/s. The dispersion profile is consistent with being flat out to the last data point. We show that a marginally-significant rise in the radial dispersion profile at a radius of 3' is not associated with any real localized kinematical substructure. Given its large distance from the Galaxy, tides are not likely to have affected the velocity dispersion, a statement we support from a quantitative kinematical analysis, as we observationally reject the occurrence of a significant apparent rotational signal or an asymmetric velocity distribution. Mass determinations adopting both isotropic stellar velocity dispersions and more general models yield a M/L ratio of 24, which is consistent with the presence of a significant dark halo with a mass of about 3x10^7 M_sun, in which the luminous component is embedded. This suggests that Leo I exhibits dark matter properties similar to those of other dSphs in the Local Group. Our data allowed us also to determine metallicities for 58 of the targets. We find a mildly metal poor mean of -1.31 dex and a full spread covering 1 dex. In contrast to the majority of dSphs, Leo I appears to show no radial gradient in its metallicities, which points to a negligible role of external influences in this galaxy's evolution.
We examine the X-ray - radio correlation in Galactic black hole sources. We highlight some of the results which extend the flux-flux relations to sources with very high accretion rates. Some of the recent results indicate that the synchrotron process is unlikely to be the mechanism responsible for the X-ray emission, particularly at high accretion rates. We present a truncated accretion disk scenario and argue that accretion rate and accretion disk geometry ultimately act as a driver of the X-ray - radio correlation. We stress the importance of wide-band X-ray spectral measurements to understand the disk-jet connection and briefly outline some attempts made in the Indian context to build instruments for wide-band X-ray spectroscopy.
In order to test the silicate-core/organic-mantle model of galactic interstellar dust, we have performed spectropolarimetry of the 3.4 micron C-H bond stretch that is characteristic of aliphatic hydrocarbons, using the nucleus of the Seyfert 2 galaxy, NGC 1068, as a bright, dusty background source. Polarization calculations show that, if the grains in NGC 1068 had the properties assigned by the core-mantle model to dust in the galactic diffuse ISM, they would cause a detectable rise in polarization over the 3.4 micron feature. No such increase is observed. We discuss modifications to the basic core-mantle model, such as changes in grain size or the existence of additional non-hydrocarbon aligned grain populations, which could better fit the observational evidence. However, we emphasize that the absence of polarization over the 3.4 micron band in NGC 1068 - and, indeed, in every line of sight examined to date - can be readily explained by a population of small, unaligned carbonaceous grains with no physical connection to the silicates.
We present the results of the analysis of RXTE (Rossi X-ray Timing Explorer) observations of the new x-ray transient, SWIFT J1753.5-0127 during its outburst in July, 2005. The source was caught at the peak of the burst with a flux of 7.19e-09 ergs-s$^{-1}$-cm$^{-2}$ in the 3-25 keV energy range and observed till it decreased by about a factor of 10. The photon index of the power law component, which is dominant during the entire outburst, decreases from $\sim$1.8 to 1.5. However, towards the end of the observations we find that the photon index is found to increase, indicating a softening of the spectra. The presence of an ultrasoft thermal component, during the bright phases of the burst, is clear from the fits to the data. The temperature associated with this thermal component is about 0.4 keV. We believe that this thermal component could be due to the presence of an accretion disk. Assuming a distance of 8.5 kpc, $L_{X}/L_{Edd} \simeq 0.05$ at the peak of the burst, for a black hole of mass $10 {\rm M}_{\odot}$. The source is found to be locked in the low/hard state during the entire outburst and likely falls in the category of the x-ray transients that are observed in the low/hard state throughout the outburst. We discuss the physical scenario of the low/hard state outburst for this source.
Motivated by an interpretation of $z \sim 3$ objects by Jimenez and Haiman (2006), we examine processes that control the fraction of primordial ($Z = 0$) gas, and so primordial stars, in high-SFR Lyman break galaxies. A primordial fraction different from 1 or 0 requires microscopic diffusion catalyzed by a velocity field with timescale comparable to the duration of star formation. The only process we found that satisfies this requirement for LBGs without fine-tuning is turbulence-enhanced mixing induced by exponential stretching and compressing of metal-rich ejecta. The time-dependence of the primordial fraction for this model is calculated. We show that conclusions for all the models discussed here are virtually independent of the IMF, including extremely top-heavy IMFs.
We have analyzed H and K_s-band images of the Arches cluster obtained using the NIRC2 instrument on Keck with the laser guide star adaptive optics (LGS AO) system. With the help of the LGS AO system, we were able to obtain the deepest ever photometry for this cluster and its neighborhood, and derive the background-subtracted present-day mass function (PDMF) down to 1.3 Msun for the 5 arcsec-9 arcsec annulus of the cluster. We find that the previously reported turnover at 6 Msun is simply due to a local bump in the mass function (MF), and that the MF continues to increase down to our 50 % completeness limit (1.3 Msun) with a power-law exponent of Gamma = -0.91 for the mass range of 1.3 < M/Msun < 50. Our numerical calculations for the evolution of the Arches cluster show that the Gamma values for our annulus increase by 0.1-0.2 during the lifetime of the cluster, and thus suggest that the Arches cluster initially had Gamma of -1.0 ~ -1.1, which is only slightly shallower than the Salpeter value.
It is currently impossible to determine the abundances of stellar populations star-by-star in dense stellar systems more distant than a few megaparsecs. Therefore, methods to analyse the composite light of stellar systems are required. I review recent progress in determining the abundances and abundance ratios of early-type galaxies. I begin with `direct' abundance measurements: colour--magnitude diagrams of and planetary nebula in nearby early-type galaxies. I then give an overview of `indirect' abundance measurements: inferences from stellar population models, with an emphasis on cross-checks with `direct' methods. I explore the variations of early-type galaxy abundances as a function of mass, age, and environment in the local Universe. I conclude with a list of continuing difficulties in the modelling that complicate the interpretation of integrated spectra and I look ahead to new methods and new observations.
We propose the Wind of Fast Rotating Massive Stars scenario to explain the origin of the abundance anomalies observed in globular clusters. We compute and present models of fast rotating stars with initial masses between 20 and 120 Msun for an initial metallicity Z=0.0005 ([Fe/H]=-1.5). We discuss the nucleosynthesis in the H-burning core of these objects and present the chemical composition of their ejecta. We consider the impact of uncertainties in the relevant nuclear reaction rates. Fast rotating stars reach the critical velocity at the beginning of their evolution and remain near the critical limit during the rest of the main sequence and part of the He-burning phase. As a consequence they lose large amounts of material through a mechanical wind which probably leads to the formation of a slow outflowing disk. The material in this slow wind is enriched in H-burning products and presents abundance patterns similar to the chemical anomalies observed in globular cluster stars. In particular, the C, N, O, Na and Li variations are well reproduced by our model. However the rate of the 24Mg(p,gamma) has to be increased by a factor 1000 around 50 MK in order to reproduce the whole amplitude of the observed Mg-Al anticorrelation. We discuss how the long-lived low-mass stars currently observed in globular clusters could have formed out of the slow wind material ejected by massive stars.
On March 2003, INTEGRAL/IBIS detected an outburst from a new source, IGR J17464-3213, that turned out to be a HEAO-1 transient, namely H1743-322. Its fux decayed below the RXTE PCA sensitivity limit in November 2003. On July 3, 2004 the source was again detected by RXTE/PCA reaching an intensity of 70 mCrab in the 2-10 keV band. Another outburst was observed in August 2005. A multiwavelength observation campaign was performed, during the three outbursts, by INTEGRAL, RXTE and CHANDRA satellites and the radio telescopes VLA and ATCA. INTEGRAL data show a good coverage during the July 2004 and August 2005 outbursts. We show here the spectral and temporal variability study performed on the data collected by INTEGRAL.
We review the nature of the oscillations of main-sequence and supergiant stars of spectral type B. Seismic tuning of the interior structure parameters of the $\beta $Cep stars has been achieved since three years. The results are based on frequencies derived from long-term monitoring and progress in this area is rapid. Oscillations in mid-B stars as well as Be stars are well established by now, but we lack good mode identification to achieve seismic modelling. We provide recent evidence of g-mode pulsations in supergiant B stars. The spherical wavenumbers of their modes are yet unidentified, preventing seismic probing of such evolved hot stars at present. Improving the situation for the three groups of g-mode oscillators requires multi-site long-term high-resolution spectroscopy in combination with either space photometry or ground-based multicolour photometry. The CoRoT programme and its ground-based programme will deliver such data in the very near future.
Cross-sections for capture reactions of charged particles in hot stellar matter turn out be increased by the quadrupole surface oscillations, if the corresponding phonon energies are of the order of the star temperature. The increase is studied in a model that combines barrier distribution induced by surface oscillations and tunneling. The capture of charged particles by nuclei with well-deformed ground-state is enhanced in stellar matter. It is found that the influence of quadrupole surface deformation on the nuclear reactions in stars grows, when mass and proton numbers in colliding nuclei increase.
In this paper, using the database of the university of Michigan Radio Astronomy Observatory (UMRAO) at three (4.8 GHz, 8 GHZ, and 14.5 GHz) radio frequencies, we studied the polarization properties for 47 BL Lacertae objects(38 radio selected BL Lacertae objects, 7 X-ray selected BL Lacertae, and two inter-middle objects (Mkn 421 and Mkn 501), and found that (1) The polarizations at higher radio frequency is higher than those at lower frequency, (2) The variability of polarization at higher radio frequency is higher than those at lower frequency, (3) The polarization is correlated with the radio spectral index, and (4) The polarization is correlated with core-dominance parameter for those objects with known core-dominance parameters suggesting that the relativistic beaming could explain the polarization characteristic of BL Lacs.
Long-time monitoring of gravitational lens systems is often done using
telescopes and recording equipment with a modest resolution. Still, it would be
interesting to get as much information as possible from the measured
lightcurves. From high resolution images we know that the recorded quasar
images are often blends and that the corresponding time series are not pure
shifted replicas of the source variability. In this paper we will develop an
algorithm to unscramble this kind of blended data.
The proposed method is based on a simple idea. We use one of the photometric
curves, which is supposedly a simple shifted replica of the source curve, to
build different artificial combined curves. Then we compare these artificial
curves with the blended curves. Proper solutions for a full set of time delays
are then obtained by varying free input parameters and estimating statistical
distances between the artificial and blended curves.
We performed a check of feasibility and applicability of the new algorithm.
For numerically generated data sets the time delay systems were recovered for a
wide range of setups. Application of the new algorithm to the classical double
quasar QSO 0957+561 A,B lightcurves shows a clear splitting of one of the
images. This is an unexpected result and extremely interesting, especially in
the context of the recent controversy about the exact time delay value for the
system.
The proposed method allows to analyse properly the data from low resolution
observations which have long time coverages. There is a number of gravitational
lens monitoring programmes, which can make use of the new algorithm.
We explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy as predicted by the magnetic reconnection model. Previous study of the radiative transfer in the photospheric region has shown that gradual dissipation of energy results in a hot photosphere. There, inverse Compton scattering of the thermal radiation advected with the flow leads to powerful photospheric emission with spectral properties close to those of the observed prompt GRB emission. Here, we build on that study by calculating the spectra for a large range of the characteristics of the flow. An accurate fitting formula is given that provides the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of the flow. It facilitates the direct comparison of the model predictions with observations, including the variability properties of the lightcurves. We verify that the model naturally accounts for the observed clustering in peak energies of the E*f(E) spectrum. In this model, the Amati relation indicates a tendency for the most luminous bursts to have more energy per baryon. If this tendency also holds for individual GRB pulses, the model predicts the observed narrowing of the width of pulses with increasing photon energy.
A study of the six largest coronal X-ray flares in the Chandra archive is presented. The flares were observed on II Peg, OU And, Algol, HR 1099, TZ CrB and CC Eri, all with the High Energy Transmission Grating spectrometer (HETG) and the ACIS detectors. We reconstruct an Emission Measure Distribution EMD(T), using a spectral line analysis method, for flare and quiescence states separately and compare the two. Subsequently, elemental abundaces are obtained from the EMD. We find similar behaviour of the EMD in all flares, namely a large high-T component appears while the low-T (kT < 2 keV) plasma is mostly unaffected, except for a small rise in the low-T Emission Measure. In five of the six flares we detect a First Ionization Potential (FIP) effect in the flare abundances relative to quiescence. This may contradict previous suggestions that flares are the cause of an inverse FIP effect in highly active coronae.
The energies of cosmic rays, fully ionized charged nuclei, extend over a wide
range up to 10^20 eV. A particularly interesting energy region spans from
10^14 to 10^18 eV, where the all-particle energy spectrum exhibits two
interesting structures, the 'knee' and the 'second knee'. An explanation of
these features is thought to be an important step in understanding of the
origin of the high-energy particles. Recent results of air shower experiments
in this region are discussed. Special attention is drawn to explain the
principle of air shower measurements - a simple Heitler model of (hadronic) air
showers is developed.
We measured low energy cosmic-ray proton and helium spectra in the kinetic energy range 0.215 - 21.5 GeV/n at different solar activities during a period from 1997 to 2002. The observations were carried out with the BESS spectrometer launched on a balloon at Lynn Lake, Canada. A calculation for the correction of secondary particle backgrounds from the overlying atmosphere was improved by using the measured spectra at small atmospheric depths ranging from 5 through 37 g/cm^2. The spectra at the top of atmosphere were determined within overall uncertainties of 12% for protons and 18% for helium nuclei, including statistical and systematic errors.
The Ks magnitude distribution of asymptotic giant branch stars provides important constraints on the spatial variation of metallicity and mean-age of the stellar population of galaxies. Here, I present results of the investigation of these parameters across the Magellanic Clouds, M33, NGC 6822 and SagDIG and I discuss the tremendous improvement that new wide-field near-infrared instruments (i.e. VISTA) will provide on our understanding of the global intrinsic as well as dynamical history of these systems.
Cold fronts have been detected both in merging and in cool core clusters, where little or no sign of a merging event is present. A systematic search of sharp surface brightness discontinuities performed on a sample of 62 galaxy clusters observed with XMM-Newton shows that cold fronts are a common feature in galaxy clusters. Indeed most (if not all) of the nearby clusters (z < 0.04) host a cold front. Understanding the origin and the nature of a such frequent phenomenon is clearly important. To gain insight on the nature of cold fronts in cool core clusters we have undertaken a systematic study of all contact discontinuities detected in our sample, measuring surface brightness, temperature and when possible abundance profiles across the fronts. We measure the Mach numbers for the cold fronts finding values which range from 0.2 to 0.9; we also detect a discontinuities in the metal profile of some clusters.
The BL Lac object S5 0716+71 was target of a coordinated and global multi-frequency campaign to search for rapid and correlated variability and signatures of the inverse-Compton catastrophe. Here we present first results obtained from a combined analysis of the cm- to sub-mm observations over a period of seven days aiming at a detailed study of the intra- to inter-day variability characteristics and to obtain constraints on the variability brightness temperatures and Doppler factors comparing the radio data with the high energy emission recorded by INTEGRAL. A more detailed description of the whole cm- to sub-mm observations and our analysis/results will be presented in a forthcoming paper. Our analysis reveals the source to be in a particular short-term variability phase when compared to the past with a correlated >~4 day time scale amplitude increase of up to 35%, which is systematically more pronounced towards higher frequencies. The obtained frequency dependent variability amplitudes and time lags contradict expectations from interstellar scintillation and strongly suggest a source intrinsic origin of this inter-day variability. A 7-day spectral evolution study indicate time-variable synchrotron self-absorption and expansion of the emission region, consistent with standard models. Assuming relativistic boosting, our different estimates of the Doppler factor yield robust lower limits of D_var,IC > 5-22 using the inverse-Compton limit and D_var,eq > 8-33 using the equipartition argument. Although high, these values are in good agreement with Doppler factors obtained from recent VLBI studies and from the inverse-Compton Doppler factors D_IC > 14-16 derived with the X-ray emission seen by INTEGRAL at 3-200 keV.
We present results from and the analysis of data from MAXIPOL, a balloon-borne experiment designed to measure the polarization in the Cosmic Microwave Background (CMB). MAXIPOL is the first CMB experiment to obtain results using a rotating half-wave plate as a rapid polarization modulator. We report results from observations of a sky area of 8 deg^2 with 10-arcmin resolution, providing information up to l~700. We use a maximum-likelihood method to estimate maps of the Q and U Stokes parameters from the demodulated time streams, and then both Bayesian and frequentist approaches to compute the EE, EB, and BB power spectra. Detailed formalisms of the analyses are given. A variety of tests show no evidence for systematic errors. The Bayesian analysis gives weak evidence for an EE signal. The EE power is 55^{+51}_{-45} \mu K^2 at the 68% confidence level for l=151-693. Its likelihood function is asymmetric and skewed positive such that with a uniform prior the probability of a positive EE power is 96%. The powers of EB and BB signals at the 68% confidence level are 18^{+27}_{-34} \mu K^2 and -31^{+31}_{-19} \mu K^2 respectively and thus consistent with zero. The upper limit of the BB-mode at the 95% confidence level is 9.5 \mu K. Results from the frequentist approach are in agreement within statistical errors. These results are consistent with the current concordance LCDM model.
We investigate newtonian description of accreting compact bodies with hard surfaces, including luminosity and selfgravitation of polytropic perfect fluids. This nonlinear integro-differential problem reduces, under appropriate boundary conditions, to an algebraic relation between luminosity and the gas abundance in stationary spherically symmetric flows. There exist, for a given luminosity, asymptotic mass and the asymptotic temperature, two sub-critical solutions that bifurcate from a critical point. They differ by the fluid content and the mass of the compact centre.
We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization of the cosmic microwave background radiation (CMB). MAXIPOL is the first bolometric CMB experiment to observe the sky using rapid polarization modulation. To build MAXIPOL, the CMB temperature anisotropy experiment MAXIMA was retrofitted with a rotating half-wave plate and a stationary analyzer. We describe the instrument, the observations, the calibration and the reduction of data collected with twelve polarimeters operating at 140 GHz and with a FWHM beam size of 10 arcmin. We present maps of the Q and U Stokes parameters of an 8 deg^2 region of the sky near the star Beta Ursae Minoris. The power spectra computed from these maps give weak evidence for an EE signal. The maximum-likelihood amplitude of l(l+1)C^{EE}_{l}/(2 pi) is 55_{-45}^{+51} uK^2 (68%), and the likelihood function is asymmetric and skewed positive such that with a uniform prior the probability that the amplitude is positive is 96%. This result is consistent with the expected concordance LCDM amplitude of 14 uK^2. The maximum likelihood amplitudes for l(l+1)C^{BB}_{l}/(2 pi) and $\ell(\ell+1)C^{EB}_{\ell}/2\pi$ are -31_{-19}^{+31} and 18_{-34}^{+27} uK^2 (68%), respectively, which are consistent with zero. All of the results are for one bin in the range 151 < l < 693. Tests revealed no residual systematic errors in the time or map domain. A comprehensive discussion of the analysis of the data is presented in a companion paper.
Based on the standard cosmological model, we calculate the correction to the rate of two-photon 2s<->1s transitions in the hydrogen atom under primordial hydrogen plasma recombination conditions that arises when the induced transitions under equilibrium background radiation with a blackbody spectrum and plasma recombination radiation are taken into account.
We report the first attempt to mode decomposition of the velocity field using wavelet transforms in the application to the magnetized turbulence analysis. We compare results of the separation with respect to the global and local mean magnetic field for models with a strong and weak external magnetic field. We show, that for models with a strong external magnetic field our results are relatively consistent for both methods. However, for superAlfv\'{e}nic turbulence with spatially variable direction of the local mean magnetic field the differences are substantial.
We report on the discovery of two spiral galaxies located behind the southern Milky Way, within the least explored region of the Great Attractor. They lie at 317, -0.5 deg galactic, where obscuration from Milky Way stars and dust exceeds 13 to 15 mag of visual extinction. The galaxies were the most prominent of a set identified using mid-infrared images of the low-latitude (|b| < 1 deg) Spitzer Legacy program GLIMPSE. Follow-up HI radio observations reveal that both galaxies have redshifts that place them squarely in the Norma Wall of galaxies, which appears to extend diagonally across the Galactic Plane from Norma in the south to Centaurus & Vela in the north. We report on the near-infrared, mid-infrared and radio properties of these newly discovered galaxies, and discuss their context in the larger view of the Great Attractor. The work presented here demonstrates that mid-infrared surveys open up a new window to study galaxies in the Zone of Avoidance.
Binary radio pulsars, first discovered by Hulse and Taylor in 1974 [1], are a unique tool for experimentally testing general relativity (GR), whose validity has been confirmed with a precision unavailable in laboratory experiments. In particular, indirect evidence of the existence of gravitational waves has been obtained. Radio pulsars in binary systems (which have come to be known as recycled) have completed the accretion stage, during which neutron star spins reach millisecond periods and their magnetic fields decay 2 to 4 orders of magnitude more weakly than ordinary radio pulsars. Among about a hundred known recycled pulsars, many have turned out to be single neutron stars. The high concentration of single recycled pulsars in globular clusters suggests that close stellar encounters are highly instrumental in the loss of the companion. A system of one recycled pulsar and one 'normal' one discovered in 2004 is the most compact among binaries containing recycled pulsars [2]. Together with the presence of two pulsars in one system, this suggests new prospects for further essential improvements in testing GR. This paper considers theoretical predictions of binary pulsars, their evolutionary formation, and mechanisms by which their companions may be lost. The use of recycled pulsars in testing GR is discussed and their possible relation to the most intriguing objects in the universe, cosmic gamma-ray bursts, is examined.
We present results of spectral and timing analysis of the fast X-ray transient XTE J1901+014 based on data of the RXTE and INTEGRAL observatories. With the INTEGRAL/ISGRI the source was detected at a significance level of 20$\sigma$ with the persistent flux of $\sim$2.7 mCrab in a 17-100 keV energy band in 2003-2004 (during long observations of the Sagittarius arm region). We added the RXTE/PCA (3-20 keV) data obtained in 1998 to the INTEGRAL/ISGRI data to build the broadband spectrum of the source in a quiescent state. It was found that the spectrum can be well approximated by a simple powerlaw with a photon index of $\sim$2.15. From timing analysis we found short time scale aperiodic variations which can be connected with instabilities in the accretion flow.
The AstroMed project at Harvard University's Initiative in Innovative
Computing (IIC) is working on improved visualization and data sharing solutions
applicable to the fields of both astronomy and medicine. The current focus is
on the application of medical imaging visualization and analysis techniques to
three-dimensional astronomical data. The 3D Slicer and OsiriX medical imaging
tools have been used to make isosurface and volumetric models in
RA-DEC-velocity space of the Perseus star forming region from the COMPLETE
Survey of Star Forming Region's spectral line maps. 3D Slicer, a brain imaging
and visualization computer application developed at Brigham and Women's
Hospital's Surgical Planning Lab, is capable of displaying volumes (i.e. data
cubes), displaying slices in any direction through the volume, generating 3D
isosurface models from the volume which can be viewed and rotated in 3D space,
and making 3D models of label maps (for example CLUMPFIND output). OsiriX is
able to generate volumetric models from data cubes and allows the user in real
time to change the displayed intensity level, crop the models without losing
the data, manipulate the model and viewing angle, and use a variety of
projections.
In applying 3D Slicer to 12CO and 13CO spectral line data cubes of Perseus,
the visualization allowed for a rapid review of over 8 square degrees and
150,000 spectra, and the cataloging of 217 high velocity points. These points
were further investigated in half of Perseus and all known outflows were
detected, and 20 points were identified in these regions as possibly being
associated with undocumented outflows. All IIC developed tools, as well as 3D
Slicer and OsiriX, are freely available.
The amount of mass loss in symbiotic systems is investigated, specifically mass loss via the formation of jets in R Aquarii (R Aqr). The jets in R Aqr have been observed in the X-ray by Chandra over a four year time period. The jet changes on times scales of a year and new outflows have been observed. Understanding the amount of mass and the frequency of ejection further constrain the ability of the white dwarf in the system to accrete enough mass to become a Type 1a supernova progenitor. The details of multi-wavelength studies, such as speed, density and spatial extent of the jets will be discussed in order to understand the mass balance in the binary system. We examine other symbiotic systems to determine trends in mass loss in this class of objects.
We investigate the morphological, dynamical, and evolutionary properties of the internetwork and network fine structure of the quiet sun at disk centre. The analysis is based on a $\sim$6 h time sequence of narrow-band filtergrams centred on the inner-wing \Ca II K$_{\rm 2v}$ reversal at 393.3 nm. The results for the internetwork are related to predictions derived from numerical simulations of the quiet sun. The average evolutionary time scale of the internetwork in our observations is 52 sec. Internetwork grains show a tendency to appear on a mesh-like pattern with a mean cell size of $\sim$4-5 arcsec. Based on this size and the spatial organisation of the mesh we speculate that this pattern is related to the existence of photospheric downdrafts as predicted by convection simulations. The image segmentation shows that typical sizes of both network and internetwork grains are in the order of 1.6 arcs.
We are undertaking a large scale radial velocity survey of the Galactic bulge which uses M giant stars selected from the 2MASS catalog as targets for the CTIO 4m Hydra multi-object spectrograph. The aim of this survey is to test dynamical models of the bulge and to quantify the importance, if any, of cold stellar streams in the bulge and its vicinity. Here we report on the kinematics of a strip of fields at -10 < l < +10 degrees and b=-4 degrees. We construct a longitude-velocity plot for the bulge stars and the model data, and find that contrary to previous studies, the bulge does not rotate as a solid body; from -5<l<+5 degrees has a slope of roughly 100 km/s/kpc and flattens considerably at greater l and reaches a maximum rotation of 45 km/s. This rotation is slower than that predicted by the dynamical model of Zhao (1996) and slower than found for planetary nebulae (Beaulieau et al. 2000). Our velocity dispersion profile is in good agreement with the Zhao model, but the slower rotation field suggests that the current model requires the inclusion of retrograde orbits. The high precision of our radial velocities, approximately 3 km/s yields an unexpected result: hints of cold kinematic features are seen in a number of the line of sight velocity distributions.
We present z band photometry of three consecutive transits of the exoplanet TrES-1, with an accuracy of 0.15% and a cadence of 40 seconds. We improve upon estimates of the system parameters, finding in particular that the planetary radius is 1.081 +/- 0.029 Jupiter radii, and the stellar radius is 0.811 +/- 0.020 solar radii. The uncertainties include both the statistical error and the systematic error arising from the uncertainty in the stellar mass. The transit times are determined to within about 15 seconds, and allow us to refine the estimate of the mean orbital period: P = 3.0300737 +/- 0.0000026 days. We find no evidence for star spots or other irregularities that have been previously reported.
We present the first self-consistent theoretical models of magnetar spectra that take into account the combined effects of the stellar atmosphere and its magnetosphere. We find that the proton cyclotron lines that are already weakened by atmospheric effects become indistinguishable from the continuum for moderate scattering optical depths in the magnetosphere. Furthermore, the hard excess becomes more pronounced due to resonant scattering and the resulting spectra closely resemble the observed magnetar spectra. We argue that while the absence of proton cyclotron lines in the observed spectra are inconclusive about the surface field strengths of magnetars, the continuum carries nearly unique signatures of the field strength and can thus be used to infer this quantity. We fit our theoretical spectra with a phenomenological two-blackbody model and compare our findings to source spectra with existing two-blackbody fits. The field strengths that we infer spectroscopically are in remarkable agreement with the values inferred from the period derivative of the sources assuming a dipole spindown. These detailed predictions of line energies and equivalent widths may provide an optimal opportunity for measuring directly the surface field strengths of magnetars with future X-ray telescopes such as {\em Constellation}-X.
We present a ~100 ksec Chandra X-ray observation and new VLA radio data of the large scale, 300 kpc long X-ray jet in PKS1127-145, a radio loud quasar at redshift z=1.18. With this deep X-ray observation we now clearly discern the complex X-ray jet morphology and see substructure within the knots. The X-ray and radio jet intensity profiles are seen to be strikingly different with the radio emission peaking strongly at the two outer knots while the X-ray emission is strongest in the inner jet region. The jet X-ray surface brightness gradually decreases by an order of magnitude going out from the core. The new X-ray data contain sufficient counts to do spectral analysis of the key jet features. The X-ray energy index of the inner jet is relatively flat with alpha_X = 0.66 +/-0.15 and steep in the outer jet with alpha_X = 1.0 +/-0.2. We discuss the constraints implied by the new data on the X-ray emission models and conclude that ``one-zone'' models fail and at least a two component model is needed to explain the jet's broad-band emission. We propose that the X-ray emission originates in the jet proper while the bulk of the radio emission comes from a surrounding jet sheath. We also consider intermittent jet activity as a possible cause of the observed jet morphology.
We promote the use of narrow-band (0.05 -- 0.20 nm FWHM) imaging in the molecularvCN band head at 388.33 nm as an effective method for monitoring small-scale magnetic field elements because it renders them with exceptionally high contrast. We create synthetic narrow-band CN filtergrams from spectra computed from a three-dimensional snapshot of a magnetohydrodynamic simulation of the solar convection to illustrate the expected high contrast and explain its nature. In addition, we performed observations with the horizontal slit spectrograph at the Dunn Solar Tower at 388.3 nm to experimentally confirm the high bright-point contrast, and to characterize and optimize the transmission profile of a narrow-band (0.04 FWHM) Lyot filter, which was built by Lyot and tailored to the CN band at Sacramento Peak in the early 70's. The presented theoretical computations predict that bright-point contrast in narrow-band (0.04 FWHM) CN filtergrams is more than 3 times higher than in CN filtergrams taken with 1 nm FWHM wide filters, and in typical G-band filtergrams.
We present near-infrared (1.0-2.4 micron) spectra confirming the youth and cool effective temperatures of 6 brown dwarfs and low mass stars with circumstellar disks toward the Chamaeleon II and Ophiuchus star forming regions. The spectrum of one of our objects indicates that it has a spectral type of ~L1, making it one of the latest spectral type young brown dwarfs identified to date. Comparing spectra of young brown dwarfs, field dwarfs, and giant stars, we define a 1.49-1.56 micron H2O index capable of determining spectral type to within 1 sub-type, independent of gravity. We have also defined an index based on the 1.14 micron sodium feature that is sensitive to gravity, but only weakly dependent on spectral type for field dwarfs. Our 1.14 micron Na index can be used to distinguish young cluster members (t <~ 5 Myr) from young field dwarfs, both of which may have the triangular H-band continuum shape which persists for at least tens of Myr. Using effective temperatures determined from the spectral types of our objects along with luminosities derived from near and mid-infrared photometry, we place our objects on the H-R diagram and overlay evolutionary models to estimate the masses and ages of our young sources. Three of our sources have inferred ages (t ~= 10-30 Myr) significantly older than the median stellar age of their parent clouds (1-3 Myr). For these three objects, we derive masses ~3 times greater than expected for 1-3 Myr old brown dwarfs with the bolometric luminosities of our sources. The large discrepancies in the inferred masses and ages determined using two separate, yet reasonable methods, emphasize the need for caution when deriving or exploiting brown dwarf mass and age estimates.
The ~800 optically unseen (R>25.5) 24mum-selected sources in the complete Spitzer First Look Survey sample (Fadda et al. 2006) with F[24mum]>0.35 mJy are found to be very strongly clustered. If, as indicated by several lines of circumstantial evidence, they are ultraluminous far-IR galaxies at z ~ [1.6-2.7], the amplitude of their spatial correlation function is very high. The associated comoving clustering length is estimated to be r_0=14.0_{-2.4}^{+2.1} Mpc, value which puts these sources amongst the most strongly clustered populations of our known universe. Their 8mum-24mum colours suggest that the AGN contribution dominates above F[24mum] ~ 0.8 mJy, consistent with earlier analyses. The properties of these objects (number counts, redshift distribution, clustering amplitude) are fully consistent with those of proto-spheroidal galaxies in the process of forming most of their stars and of growing their active nucleus, as described by the Granato et al. (2004) model. In particular, the inferred space density of such galaxies at z ~ 2 is much higher than what expected from most semi-analytic models. Matches of the observed projected correlation function w(\theta) with models derived within the so-called Halo Occupation Scenario show that these sources have to be hosted by haloes more massive than ~10^{13.4} M_\odot. This value is significantly higher than that for the typical galactic haloes hosting massive elliptical galaxies, suggesting a duration of the starburst phase of massive high-redshift dusty galaxies of T_B ~ 0.5 Gyr.
Interferometric observation of the CMB polarization can be expressed as a linear sum of spherical harmonic coefficients $a_{\pm 2,lm}$ of the CMB polarization. The linear weight for $a_{\pm 2,lm}$ depends on the observational configuration such as antenna pointing, baseline orientation, and spherical harmonic number $l,m$. Since an interferometer is sensitive over a finite range of multipoles, $a_{\pm 2,lm}$ in the range can be determined by fitting $a_{\pm 2,lm}$ for visibilities of various observational configurations. The formalism presented in this paper enables the determination of $a_{\pm 2,lm}$ directly from spherical harmonic spaces without spherical harmonic transformation of pixellized maps. The result of its application to a simulated observation is presented with the formalism.
A recent study suggests that the quasar HE 0450-2958 is hosted by a galaxy substantially fainter than that inferred from the correlation between black hole mass and bulge luminosity. As this result has significant bearings on galaxy and black hole evolution, we revisit the issue by performing an independent analysis of the data, using a two-dimensional image fitting technique. We indeed find no evidence of a host galaxy either, but, due to the brightness of the quasar and uncertainties in the point-spread function, the limits are fairly weak. To derive an upper limit on the host galaxy luminosity, we perform simulations to deblend the quasar from the host under conditions similar to those actually observed. We find that the host galaxy has an absolute magnitude upper limit of -20 < M_V < -21, in good agreement with the previous determination. Since this limit is consistent with the value predicted from the current best estimate of the black hole mass, there is no compelling evidence that the quasar HE 0450-2958 has an abnormally underluminous host galaxy. We also show that, contrary to previous claims, the companion galaxy to HE 0450-2958 should not be be regarded as an ultraluminous infrared galaxy.
Many clues about the galaxy assembly process lurk in the faint outer regions of galaxies. Although quantitative study of these parts has been severely limited in the past, breakthroughs are now being made thanks to the combination of wide-area star counts, deep HST imagery and 8-m class spectroscopy. I highlight here some recent progress made on deciphering the fossil record encoded in the outskirts of our nearest large neighbours, M31 and M33.
Thanks to aggressive campaigns of multi-wavelength observations of X-ray binaries in outbursts over the last decade or so, we have now reached a reasonable understanding of their radio phenomenology in response to changes in the global X-ray properties. Here I shall subjectively review the latest progresses made in assessing the interplay between inflow and outflow from an observational point of view, as well as point out a number of open issues that still need to be addressed both theoretically and observationally.
The robotic ROTSE-III telescope network detected prompt optical emission contemporaneous with the gamma-ray emission of Swift events GRB051109A and GRB051111. Both datasets have continuous coverage at high signal-to-noise levels from the prompt phase onwards, thus the early observations are readily compared to the Swift XRT and BAT high energy detections. In both cases, the optical afterglow is established, declining steadily during the prompt emission. For GRB051111, there is evidence of an excess optical component during the prompt emission. The component is consistent with the flux spectrally extrapolated from the gamma-rays, using the gamma-ray spectral index. A compilation of spectral information from previous prompt detections shows that such a component is unusual. The existence of two prompt optical components - one connected to the high-energy emission, the other to separate afterglow flux, as indicated in GRB051111 - is not compatible with a simple ``external-external'' shock model for the GRB and its afterglow.
Recently the CO line width (FWHM(CO)) has been suggested to be a surrogate of the bulge velocity dispersion ($\sigma$) of the host galaxies of high redshift quasars, and the black hole -- bulge ($M_{BH}-\sigma$) relation obtained with this assumption departs significantly from the $M_{BH}-\sigma$ relation in the local universe. In this study we first present an investigation on the correlation between the CO line width and the bulge velocity dispersion using a sample of 33 nearby Seyfert galaxies. We find that the formula adopted in previous studies, $\sigma=\rm{FWHM(CO)}/2.35$, is generally not a good approximation. Using it one may underestimate the value of bulge velocity dispersion significantly when the CO line is narrower than 400 $km s^{-1}$. By involving the galactic inclination angle $i$ as an additional parameter, we obtain a tight correlation between the inclination-corrected CO line width and the bulge velocity dispersion, namely, $\rm {FWHM(CO)}/\sin i=-67.16\pm80.18+(3.62\pm0.68)\sigma$. Using this new relation, we can better estimate the bulge velocity dispersion from the CO line width if the galactic inclination is known. We apply this new relation to 9 high-redshift quasars with CO line detections, and find that they are consistent with the local $M_{BH}-\sigma$ relation if their inclination angles are around $15^o$. The possible smaller inclinations of the high redshift quasars are preferred because of the detections of themselves, and are also consistent with their relatively smaller CO line widths in comparison with submillimeter galaxies (SMGs) at high redshift with the similar total amount of molecular gas. Future observations are needed to confirm these results.
This paper describes a novel fourth-order integration algorithm for the gravitational N-body problem based on discrete Lagrangian mechanics. When used with shared timesteps, the algorithm is momentum conserving and symplectic. We generalize the algorithm to handle individual time steps; this introduces fifth-order errors in angular momentum conservation and symplecticity. We show that using block power of two timesteps does not increase the error in symplecticity. In contrast to other high-order, symplectic, momentum-preserving algorithms in widespread astrophysical use, the algorithm takes only forward timesteps. We compare a code integrating an N-body system using the algorithm with a direct-summation force calculation to standard stellar cluster simulation codes. We find that our algorithm has significantly better symplecticity and momentum conservation errors than standard algorithms for equivalent numbers of potential evaluations and equivalent energy conservation errors.
High-dispersion echelle spectroscopy in optical forbidden lines of O^0, S^+, and S^2+ is used to construct velocity-resolved images and electron density maps of the inner region of the Orion nebula with a resolution of 10 km s-1 x 3" x 2". Among the objects and regions newly discovered in this study are (1) the Diffuse Blue Layer: an extended layer of moderately blue-shifted, low-density, low-ionization emission in the southeast region of the nebula; (2) the Red Bay: a region to the east of the Trapezium where the usual correlation between velocity and ionization potential is very weak, and where the emitting layer is very thick; and (3) HH 873: a new redshifted jet to the southwest of the Trapezium.
Using the ``modified DPMJET-III'' model explained in the previous paper, we calculate the atmospheric neutrino flux. The calculation scheme is almost the same as HKKM04 \cite{HKKM2004}, but the usage of the ``virtual detector'' is improved to reduce the error due to it. Then we study the uncertainty of the calculated atmospheric neutrino flux summarizing the uncertainties of individual components of the simulation. The uncertainty of $K$-production in the interaction model is estimated by modifying FLUKA'97 and Fritiof 7.02 so that they also reproduce the atmospheric muon flux data correctly, and the calculation of the atmospheric neutrino flux with those modified interaction models. The uncertainties of the flux ratio and zenith angle dependence of the atmospheric neutrino flux are also studied.
Observations give evidences of the presence of metals in the intergalactic medium (IGM). The stars responsible for transforming hydrogen and helium into more complex atoms do not form outside the galaxies in the standard scenario of galaxy formation. Supernovae-driven winds and their associated feedback was proposed as a possible solution to explain such enrichment of the IGM. It turned out that a proper modelling of supernovae explosions within a turbulent interstellar medium (ISM) is a difficult task. Recent advances have been obtained using a multiphase approach to solve for the thermal state of the ISM, plus some additional recipes to account for the kinetic effect of supernovae on the galactic gas. We briefly describe here our implementation of supernovae feedback within the RAMSES code, and apply it to the formation and evolution of isolated galaxies of various masses and angular momenta. We have explored under what conditions a galactic wind can develop, if one considers only a quiescent mode of star formation. We have also characterized the distribution and evolution of metallicity in the gas outflow spreading in the IGM.
As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) program, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph's): Sculptor, Sextans, Fornax, and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below [Fe/H] ~ -3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSph's appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.
Hydrodynamical simulations of star formation indicate that the motions of protostars through their natal molecular clouds may be crucial in determining the properties of stars through competitive accretion and dynamical interactions. Walsh, Myers & Burton recently investigated whether such motions might be observable in the earliest stages of star formation by measuring the relative shifts of line-centre velocities of low- and high-density tracers of low-mass star-forming cores. They found very small (~0.1 km/s) relative motions. In this paper, we analyse the hydrodynamical simulation of Bate, Bonnell & Bromm and find that it also gives small relative velocities between high-density cores and low-density envelopes, despite the fact that competitive accretion and dynamical interactions occur between protostars in the simulation. Thus, the simulation is consistent with the observations in this respect. However, we also find some differences between the simulation and the observations. Overall, we find that the high-density gas has a higher velocity dispersion than that observed by Walsh et al. We explore this by examining the dependence of the gas velocity dispersion on density and its evolution with time during the simulation. We find that early in the simulation the gas velocity dispersion decreases monotonically with increasing density, while later in the simulation, when the dense cores have formed multiple objects, the velocity dispersion of the high-density gas increases. Thus, the simulation is in best agreement with the observations early on, before many objects have formed in each dense core.
Techniques to extract information from spectra of unresolved multi-component systems are revised, with emphasis on recent developments and practical aspects. We review the cross-correlation techniques developed to deal with such spectra, discuss the determination of the broadening function and compare techniques to reconstruct component spectra. The recent results obtained by separating or disentangling the component spectra is summarized. An evaluation is made of possible indeterminacies and random and systematic errors in the component spectra.
We present the results of a 21 cm neutral hydrogen (HI) line detection experiment in the direction of 18 low luminosity dwarf galaxies of the Centaurus A group, using the Australia Telescope National Facility 64m Parkes Radio Telescope and the Australia Telescope Compact Array. Five dwarfs have HI masses between M_HI=4x10^5 to M_HI=2.1x10^7 Msol and 0.04<M_HI/L_B<1.81 Msol L_{sol, B}^-1. The other 13 have upper-limits between M_HI<5x10^5 and M_HI<4x10^6 Msol (M_HI}/L_B<0.24 Msol L_{sol, B}^-1). Two of the mixed-morphology dwarfs remain undetected in HI, a situation that is in contrast to that of similar Local Group and Sculptor group objects where all contain significant amounts of neutral gas. There is a discontinuity in the HI properties of Centaurus A group low luminosity dwarfs that is unobserved amongst Sculptor group dwarfs. All objects fainter than M_B=-13 have either M_HI>10^7 Msol or M_HI<10^6 Msol. This gap may be explained by the ram pressure stripping mechanism at work in this dense environment where all galaxies with M_HI<10^7 Msol have been stripped of their gas. The required intergalactic medium density to achieve this is ~10^-3 cm^-3.
Context. Wolf Rayet/black hole binaries are believed to exist as a later evolutionary product of high-mass X-ray binaries. Hundreds of such binaries may exist in galaxies, but only a few of them are close enough to be observed as X-ray binaries. Only a couple of candidates have been reported so far. Aims. Based on XMM-Newton observations, we report the positional coincidence of the brightest X-ray source in NGC 300 (NGC 300 X-1) with a Wolf-Rayet candidate. Temporal and spectral analysis of the X-ray source is performed. Methods. We determine an accurate X-ray position of the object, and derive light curves, spectra and flux in four XMM-Newton observations. Results. The positions of the X-ray source and the helium star candidate coincide within 0.11"+-0.45". The X-ray light curves show irregular variability. During one XMM-Newton observation, the flux increased by about a factor of ten in 10 hours. The spectrum can be modelled by a power-law with Gamma~2.45 with additional relatively weak line emission, notably around 0.95 kev. The mean observed (absorbed) luminosity in the 0.2-10 kev band is ~2x10^38 erg/s. Conclusions. NGC 300 X-1 is a good candidate for a Wolf-Rayet/black-hole X-ray binary: its position coincides with a Wolf-Rayet candidate and the unabsorbed X-ray luminosity reached L(0.2-10 kev)~1x10^39 erg/s, suggesting the presence of a black hole.
Based on the VLBI Space Observatory Programme (VSOP) observations at 1.6 and 5 GHz, we find that the luminous high-redshift (z=3.215) quasar PKS 1402+044 (J1405+0415) has a pronounced 'core--jet' structure. The jet shows a steeper spectral index and lower brightness temperature with the increase of the distance from the core. The variation of brightness temperature is basically consistent with the shock-in-jet model. Assuming that the jet is collimated by the ambient magnetic field, we estimate the mass of the central object as ~10^9 M_sun. The upper limit of the jet proper motion of PKS 1402+044 is 0.03 mas/yr (~3c) in the east-west direction.
(Context) When a galaxy acquires material from the outside, it is likely that the resulting angular momentum of the accreted material is decoupled from that of the pre-existing galaxy. The presence of stars counter-rotating with respect to other stars and/or gas represents an extreme case of decoupling. (Aims) NGC5719, an almost edge-on Sab galaxy with a prominent skewed dust lane, shows a spectacular on-going interaction with its face-on Sbc companion NGC5713. Observations of such interacting systems provide insight into the processes at work in assembling and reshaping galaxies. (Methods) Studies were made of the distribution and kinematics of neutral hydrogen in the NGC5719/13 galaxy pair and the ionised gas and stellar kinematics along the major axis of NGC5719. (Results) Two HI tidal bridges that loop around NGC5719 and connect to NGC5713, and two HI tidal tails departing westward from NGC5713 were detected. There is a correspondence between the HI condensations and the location of clumps of young stars within and outside the disc of NGC5719. The low-mass satellite PGC135857 at the tip of the northern tail was detected in HI, and is likely a by-product of the interaction. The neutral and ionised hydrogen in the disc of NGC5719 are counter-rotating with respect to the main stellar disc. The counter-rotating stellar disc contains about 20% of the stars in the system, and has the same radial extension as the main stellar disc. This is the first interacting system in which a counter-rotating stellar disc has been detected. (Conclusions) The data support a scenario where HI from the large reservoir available in the galaxy's surroundings was accreted by NGC5719 onto a retrograde orbit and subsequently fuelled the in-situ formation of the counter-rotating stellar disc.
The star gamma Cep is known as a single-lined spectroscopic triple system at a distance of 13.8 pc, composed of a K1 III-IV primary star with V = 3.2 mag, a stellar-mass companion in a 66--67 year orbit (Torres 2006), and a substellar companion with M_p sin i = 1.7 M_Jup that is most likely a planet (Hatzes et al. 2003). We aim to obtain a first direct detection of the stellar companion, to determine its current orbital position (for comparison with the spectroscopic and astrometric data), its infrared magnitude and, hence, mass. We use the Adaptive Optics camera CIAO at the Japanese 8m telescope Subaru on Mauna Kea, Hawaii, with the semi-transparent coronograph to block most of the light from the bright primary gamma Cep A, and to detect at the same time the faint companion B. In addition, we also used the IR camera Omega-Cass at the Calar Alto 3.5m telescope, Spain, to image gamma Cep A and B by adding up many very short integrations (without AO). gamma Cep B is clearly detected on our CIAO and Omega-Cass images. We use a photometric standard star to determine the magnitude of B after PSF subtraction in the Subaru image, and the magnitude difference between A and B in the Calar Alto images, and find an average value of K = 7.3 \pm 0.2 mag. The separations and position angles between A and B are measured on 15 July 2006 and 11 and 12 Sept 2006, B is slightly south of west of A. By combining the radial velocity, astrometric, and imaging data, we have refined the binary orbit and determined the dynamical masses of the two stars in the gamma Cep system, namely 1.40 \pm 0.12 M_sun for the primary and 0.409 \pm 0.018 M_sun for the secondary (consistent with being a M4 dwarf). We also determine the minimum mass of the sub-stellar companion to be M_p sin i = 1.60 \pm 0.13 M_Jup.
A class of exact general relativistic rotating thick disks surrounded by rotating dust is constructed by applying the ``displace, cut, fill and reflect'' method on the van Stockum class of metrics. Two particular solutions are considered in detail. We find that the disks have annular regions with negative energy density and heat flow is present in most of the radial extension of the disk. It is shown that thin disks are composed of exotic matter with an equation of state of cosmic strings or struts. We also comment the recent relativistic galactic model proposed by Cooperstock and Tieu and the presence of an additional disk of exotic matter in their model.
We report on mid- and far-IR Spitzer observations of 7 nearby dusty elliptical galaxies by using the Multiband Imaging Photometer (MIPS) and Infrared Spectrograph (IRS). Our sample galaxies are known to contain an excessive amount of interstellar dust against sputtering destruction in hot plasma filling the interstellar space of elliptical galaxies. In order to study the origin and the properties of the excess dust in the hot plasma, we selected galaxies with a wide range of X-ray luminosities but similar optical luminos ities for our Spitzer Guest Observers (GO1) program. The 7 galaxies are detected at the MIPS 24 um, 70 um, and 160 um bands; the far- to mid-IR flux ratios of relatively X-ray-bright elliptical galaxies are lower than those of X-ray-faint galaxies. From the IRS spectra, polycyclic aromatic hydrocarbon (PAH) emission features are detected significantly from 5 of the 7 galaxies; t he emission intensities are weaker as the X-ray luminosity of the galaxy is larger. We have found a correlation between the far- to mid-IR flux ratio and the equivalent width of the PAH emission feature. We have obtained apparent spatial correspondence between mid-IR and X-ray distributions in the outer regions for the three X-ray-brightest galaxies in our sample. Possible interpretations for our observational results are discussed.
We develop a model for the wind properties of cool main-sequence stars, which comprises their wind ram pressures, mass fluxes, and terminal wind velocities. The wind properties are determined through a polytropic magnetised wind model, assuming power laws for the dependence of the thermal and magnetic wind parameters on the stellar rotation rate. We use empirical data to constrain theoretical wind scenarios, which are characterised by different rates of increase of the wind temperature, wind density, and magnetic field strength. Scenarios based on moderate rates of increase yield wind ram pressures in agreement with most empirical constraints, but cannot account for some moderately rotating targets, whose high apparent mass loss rates are inconsistent with observed coronal X-ray and magnetic properties. For fast magnetic rotators, the magneto-centrifugal driving of the outflow can produce terminal wind velocities far in excess of the surface escape velocity. Disregarding this aspect in the analyses of wind ram pressures leads to overestimations of stellar mass loss rates. The predicted mass loss rates of cool main-sequence stars do not exceed about ten times the solar value. Our results are in contrast with previous investigations, which found a strong increase of the stellar mass loss rates with the coronal X-ray flux. Owing to the weaker dependence, we expect the impact of stellar winds on planetary atmospheres to be less severe and the detectability of magnetospheric radio emission to be lower then previously suggested. Considering the rotational evolution of a one solar-mass star, the mass loss rates and the wind ram pressures are highest during the pre-main sequence phase.
The Monitor project is a photometric monitoring survey of nine young
(1-200Myr) clusters in the solar neighbourhood to search for eclipses by very
low mass stars and brown dwarfs and for planetary transits in the light curves
of cluster members. It began in the autumn of 2004 and uses several 2 to 4m
telescopes worldwide. We aim to calibrate the relation between age, mass,
radius and where possible luminosity, from the K-dwarf to the planet regime, in
an age range where constraints on evolutionary models are currently very
scarce. Any detection of an exoplanet in one of our youngest targets (<=10Myr)
would also provide important constraints on planet formation and migration
timescales and their relation to proto-planetary disc lifetimes. Finally, we
will use the light curves of cluster members to study rotation and flaring in
low-mass pre-main sequence stars.
The present paper details the motivation, science goals and observing
strategy of the survey. We present a method to estimate the sensitivity and
number of detections expected in each cluster, using a simple semi-analytic
approach which takes into account the characteristics of the cluster and
photometric observations, using (tunable) best-guess assumptions for the
incidence and parameter distribution of putative companions, and we incorporate
the limits imposed by radial velocity follow-up from medium and large
telescopes. We use these calculations to show that the survey as a whole can be
expected to detect over 100 young low and very low mass eclipsing binaries, and
approx. 3 transiting planets with radial velocity signatures detectable with
currently available facilities.
By employing a local two-fluid theory, we investigate an obliquely propagating electromagnetic instability in the lower hybrid frequency range driven by cross-field current or relative drifts between electrons and ions. The theory self-consistently takes into account local cross-field current and accompanying pressure gradients. It is found that the instability is caused by reactive coupling between the backward propagating whistler (fast) waves in the moving electron frame and the forward propagating sound (slow) waves in the ion frame when the relative drifts are large. The unstable waves we consider propagate obliquely to the unperturbed magnetic field and have mixed polarization with significant electromagnetic components. A physical picture of the instability emerges in the limit of large wavenumber characteristic of the local approximation. The primary positive feedback mechanism is based on reinforcement of initial electron density perturbations by compression of electron fluid via induced Lorentz force. The resultant waves are qualitatively consistent with the measured electromagnetic fluctuations in reconnecting current sheet in a laboratory plasma.
We combine radial velocities, proper motions, and low resolution abundances for a sample of 315 K and M giants in the Baade's Window (l,b)=(0.9,-4) Galactic bulge field. The velocity ellipsoid of stars with [Fe/H]>-0.5 dex shows a vertex deviation in the plot of radial versus transverse velocity, consistent with that expected from a population with orbits supporting a bar. We demonstrate that the significance of this vertex deviation using non-parametric rank correlation statistic is >99%. The velocity ellipsoid for the metal poor ([FeH]<-0.5) part of the population shows no vertex deviation and is consistent with an isotropic, oblate rotating population. We find no evidence for kinematic subgroups, but there is a mild tendency for the vertical velocity dispersion sigma_b to decrease with increasing metallicity.
AIMS: After the first dredge-up, low-mass Red Giant Branch (RGB) stars experience an extra-mixing episode that strongly affects the chemical abundances on their surface. This mixing occurs at the bump in the luminosity function. In this Letter we describe the efficiency of the extra-mixing in RGB stars found in very metal-poor globular clusters (GC) METHODS: The VLT/ISAAC spectra of twenty stars located between the bump and the tip of the RGB in four GCs with metallicities between [Fe/H]=-1.2 and -2.5dex were collected. The carbon isotopic ratios on their surface were derived from the second overtone bands of the CO molecule at 2.3mic with the spectral synthesis method. RESULTS: It is found that the carbon isotopic ratios of very metal-poor GC stars always reach the equilibrium value of the CNO cycle almost immediately above the bump in the luminosity function. No additional mixing episode at brighter luminosities and no variations with the clusters' metallicity were detected. The extra-mixing is therefore found to be very efficient in metal-poor low-mass RGB stars, in very good agreement with theoretical expectations.
An exponentially expanding Universe, possibly governed by a cosmological constant, forces gravitationally bound structures to become more and more isolated, eventually becoming causally disconnected from each other and forming so-called "island universes". This new scenario reformulates the question about which will be the largest structures that will remain gravitationally bound, together with requiring a systematic tool that can be used to recognize the limits and mass of these structures from observational data, namely redshift surveys of galaxies. Here we present a method, based on the spherical collapse model and N-body simulations, by which we can estimate the limits of bound structures as observed in redshift space. The method is based on a theoretical criterion presented in a previous paper that determines the mean density contrast that a spherical shell must have in order to be marginally bound to the massive structure within it. Understanding the kinematics of the system, we translated the real-space limiting conditions of this "critical" shell to redshift space, producing a projected velocity envelope that only depends on the density profile of the structure. From it we created a redshift-space version of the density contrast that we called "density estimator", which can be calibrated from N-body simulations for a reasonable projected velocity envelope template, and used to estimate the limits and mass of a structure only from its redshift-space coordinates.
Aims: The binary pulsar PSR J1811-1736 has been identified, since its discovery, as a member of a double neutron star system. Observations of such binary pulsars allow the measurement of general relativistic effects, which in turn lead to information about the orbiting objects and, in a few cases, to tests of theories of gravity. Methods: Regular timing observations have been carried out with three of the largest European radio telescopes involved in pulsar research. The prospects of continued observations were studied with simulated timing data. Pulse scattering times were measured using dedicated observations at 1.4 GHz and at 3.1 GHz, and the corresponding spectral index has also been determined. The possibility of detecting the yet unseen companion as a radio pulsar was also investigated. A study of the natal kick received by the younger neutron star at birth was performed. Results: We present an up to date and improved timing solution for the binary pulsar PSR J1811-1736. One post-Keplerian parameter, the relativistic periastron advance, is measured and leads to the determination of the total mass of this binary system. The pulse profile at 1.4 GHz is heavily broadened by interstellar scattering, limiting the timing precision achievable at this frequency and the measurability of other post-keplerian parameters. Interstellar scattering is unlikely to be the reason for the continued failure to detect radio pulsations from the companion of PSR J1811-1736. The probability distribution that we derive for the amplitude of the kick imparted on the companion neutron star at its birth indicates that the kick has been of low amplitude.
The Cartwheel is one of the most outstanding examples of a dynamically perturbed galaxy where star formation is occurring inside the ring--like structure. In previous studies with Chandra, we detected 16 Ultra Luminous X-ray sources lying along the southern portion of the ring. Their Luminosity Function is consistent with them being in the high luminosity tail of the High Mass X-ray Binaries distribution, but with one exception: source N.10. This source, detected with Chandra at L_X = 1.x 10^(41) ergs, is among the brightest non--nuclear sources ever seen in external galaxies. Recently, we have observed the Cartwheel with XMM-Newton in two epochs, six months apart. After having been at its brightest for at least 4 years, the source has dimmed by at least a factor of two between the two observations. This fact implies that the source is compact in nature. Given its extreme isotropic luminosity, there is the possibility that the source hosts an accreting intermediate--mass black hole. Other sources in the ring vary in flux between the different datasets. We discuss our findings in the context of ULX models.
The goal of this work is to study the incidence rate of "cooling flows" in the high redshift clusters using Chandra observations of z>0.5 objects from a new large, X-ray selected catalog. We find that only a very small fraction of high-z objects have cuspy X-ray brightness profiles, which is a characteristic feature of the cooling flow clusters at z~0. The observed lack of cooling flows is most likely a consequence of a higher rate of major mergers at z>0.5.
We study the time dependent spectra produced via the bulk Compton process by a cold, relativistic shell of plasma moving (and accelerating) along the jet of a blazar, scattering on external photons emitted by the accretion disc and reprocessed in the broad line region. Bulk Comptonization of disc photons is shown to yield a spectral component contributing in the far UV band, and would then be currently unobservable. On the contrary, the bulk Comptonization of broad line photons may yield a significant feature in the soft X-ray band. Such a feature is time-dependent and transient, and dominates over the non thermal continuum only when: a) the dissipation occurs close to, but within, the broad line region; b) other competing processes, like the synchrotron self-Compton emission, yield a negligible flux in the X-ray band. The presence of a bulk Compton component may account for the X-ray properties of high redshift blazars that show a flattening (and possibly a hump) in the soft X-rays, previously interpreted as due to intrinsic absorption. We discuss why the conditions leading to a detectable bulk Compton feature might be met only occasionally in high redshift blazars, concluding that the absence of such a feature in the spectra of most blazars should not be taken as evidence against matter--dominated relativistic jets. The detection of such a component carries key information on the bulk Lorentz factor and kinetic energy associated to (cold) leptons.
Recent observations of long and short gamma-ray bursts have revealed a puzzling X-ray activity that in some cases continuous until hours after the burst. It is difficult to reconcile such time scales with the viscous time scales that an accretion disk can plausibly provide. Here I discuss the accretion activity expected from the material that during a compact binary merger coalescence is launched into eccentric, but gravitationally bound orbits. From a simple analytical model the time scales and accretion luminosities that result from fallback in the aftermath of a compact binary merger are derived. For the considered mass range double neutron star binaries are relatively homogeneous in their fallback luminosities, neutron star black hole systems show a larger spread in their fallback behaviour. While the model is too simple to make predictions about the detailed time structure of the fallback, it is expected to make reasonable predictions about its gross properties. About one hour after the coalescence the fallback accretion luminosity can still be as large as $\sim 10^{45}$ erg/s, a fraction of which will be transformed into X-rays. Large-scale amplitude variations in the X-ray luminosities can plausibly be caused by gravitational fragmentation, which for the high-eccentricity fallback should occur more easily than in an accretion disk.
The evolution of laboratory produced magnetic jets is followed numerically through three-dimensional, non-ideal magnetohydrodynamic simulations. The experiments are designed to study the interaction of a purely toroidal field with an extended plasma background medium. The system is observed to evolve into a structure consisting of an approximately cylindrical magnetic cavity with an embedded magnetically confined jet on its axis. The supersonic expansion produces a shell of swept-up shocked plasma which surrounds and partially confines the magnetic tower. Currents initially flow along the walls of the cavity and in the jet but the development of current-driven instabilities leads to the disruption of the jet and a re-arrangement of the field and currents. The top of the cavity breaks-up and a well collimated, radiatively cooled, 'clumpy' jet emerges from the system.
Several relatively bright, persistent X-ray sources display regular pulses, with periods in the range of 700-10000 s. These sources are identified with massive close binaries in which a neutron star accretes material onto its surface. The observed pulsations in all of them, but one, are unambiguously associated with the spin period of the neutron star. Analyzing possible history of these pulsars I conclude that the neutron stars in these systems undergo spherical accretion and their evolutionary tracks in a previous epoch contained three instead of two states, namely, ejector, supersonic propeller, and subsonic propeller. An assumption about a supercritical value of the initial magnetic field of the neutron stars within this scenario is not necessary. Furthermore, I show that the scenario in which the neutron star in 2S 0114+650 is assumed to be a magnetar descendant encounters major difficulties in explaining the evolution of the massive companion. An alternative interpretation of the spin evolution of the neutron star in this system is presented and the problem raised by association of the 10000 s pulsations with the spin period of the neutron star is briefly discussed.
The Visible-light Imager and Magnetograph (VIM) proposed for the ESA Solar Orbiter mission will observe a photospheric spectral line at high spatial resolution. Here we simulate and interpret VIM measurements. Realistic MHD models are used to synthesize "observed" Stokes profiles of the photospheric Fe I 617.3 nm line. The profiles are degraded by telescope diffraction and detector pixel size to a spatial resolution of 162 km on the solar surface. We study the influence of spectral resolving power, noise, and limited wavelength sampling on the vector magnetic fields and line-of-sight velocities derived from Milne-Eddington inversions of the simulated measurements. VIM will provide reasonably accurate values of the atmospheric parameters even with filter widths of 120 mA and 3 wavelength positions plus continuum, as long as the noise level is kept below 10^-3 I_c.
Over the past several years, numerous examples of X-ray cavities coincident with radio sources have been observed in so-called "cool core" clusters of galaxies. Motivated by these observations, we explore the evolution and the effect of cavities on a cooling intracluster medium (ICM) numerically, adding relevant physics step by step. In this paper we present a first set of hydrodynamical, high resolution (1024^3 effective grid elements), three-dimensional simulations, together with two-dimensional test cases. The simulations follow the evolution of radio cavities, modeled as bubbles filled by relativistic plasma, in the cluster atmosphere while the ICM is subject to cooling. We find that the bubble rise retards the development of a cooling flow by inducing motions in the ICM which repeatedly displace the material in the core. Even bubbles initially set significantly far from the cluster center affect the cooling flow, although much later than the beginning of the simulation. The effect is, however, modest: the cooling time is increased by at most only 25%. As expected, the overall evolution of pure hydrodynamic bubbles is at odds with observations, showing that some additional physics has to be considered in order to match the data.
The main aim is to investigate the possibility of a high frequency turn-over in the radio spectrum of pulsars. Using the GMRT, multi-frequency flux density measurements of several candidate pulsars have been carried out and their spectra have been extended to lower frequencies. We present the first direct evidence for turn-over in pulsar radio spectra at high frequencies. A total of 3 pulsars (including 2 new ones from this study) are now shown to have a turn-over frequency > 1 GHz, and one is shown to have a turn-over at ~600 MHz.
The iron depletion level and the gas-phase iron abundance in the hot ~10^6 K interstellar medium (ISM) is critical to the understanding of its energy balance as well as the thermal sputtering, cooling, and heating processes of dust grains. Here we report on the first detection of the Fe XVII absorption line at 15.02 A from the hot ISM in the spectrum of the low mass X-ray binary 4U~1820-303 observed with Chandra X-ray observatory. By jointly analyzing this absorption line with those from OVII, OVIII, and NeIX ions in the same spectrum, we obtain an abundance ratio as Fe/Ne=0.8(0.4, 2.1) in units of Anders & Grevesse's solar value (90% confidence intervals). We find that the result is robust with respect to different assumed gas temperature distributions. The obtained Fe/Ne abundance ratio, albeit with large uncertainties, is consistent with the solar value, indicating that there is very little or no iron depleted into dust grains, i.e., most of or all of the dust grains have been destroyed in the hot ISM.
We use spectrally dispersed near-IR interferometry data to constrain the temperature profiles of sub-AU-sized regions of 11 Herbig Ae/Be sources. We find that a single-temperature ring does not reproduce the data well. Rather, models incorporating radial temperature gradients are preferred. These gradients may arise in a dusty disk, or may reflect separate gas and dust components with different temperatures and spatial distributions. Comparison of our models with broadband spectral energy distributions suggests the latter explanation. The data support the view that the near-IR emission of Herbig Ae/Be sources arises from hot circumstellar dust and gas in sub-AU-sized disk regions. Intriguingly, our derived temperature gradients appear systematically steeper for disks around higher mass stars. It is not clear, however, whether this reflects trends in relative dust/gas contributions or gradients within individual components.
The gravitational microlensing light curves that reveal the presence of extrasolar planets generally yield the planet-star mass ratio and separation in units of the Einstein ring radius. The microlensing method does not require the detection of light from the planetary host star. This allows the detection of planets orbiting very faint stars, but it also makes it difficult to convert the planet-star mass ratio to a value for the planet mass. We show that in many cases, the lens stars are readily detectable with high resolution space-based follow-up observations. When the lens star is detected, the lens-source relative proper motion can also be measured, and this allows the masses of the planet and its host star to be determined and the star-planet separation can be converted to physical units.For the recently detected super-Earth planet, OGLE-2005-BLG-169Lb, we show that the lens star will definitely be detectable with observations by the Hubble Space Telescope (HST) unless it is a stellar remnant. Finally, we show that most planets detected by a space-based microlensing survey are likely to orbit host stars that will be detected and characterized by the same survey.
We report the results of a survey for fluorescent Ly-alpha emission carried out in the field surrounding the z=3.1 quasar QSO0420-388 using the FORS2 instrument on the VLT. We first review the properties expected for fluorescent Ly-alpha emitters, compared with those of other non-fluorescent Ly-alpha emitters. Our observational search detected 13 Ly-alpha sources sparsely sampling a volume of ~14000 comoving Mpc^3 around the quasar. The properties of these in terms of i) the line equivalent width, ii) the line profile and iii) the value of the surface brightness related to the distance from the quasar, all suggest that several of these may be plausibly fluorescent. Moreover, their number is in good agreement with the expectation from theoretical models. One of the best candidates for fluorescence is sufficiently far behind QSO0420-388 that it would imply that the quasar has been active for (at least) ~60 Myrs. Further studies on such objects will give information about proto-galactic clouds and on the radiative history (and beaming) of the high-redshift quasars.
We report the results of simultaneous observations of the Vela pulsar in X-rays and radio from the RXTE satellite and the Mount Pleasant Radio Observatory in Tasmania. We sought correlations between the Vela's X-ray emission and radio arrival times on a pulse by pulse basis. At a confidence level of 99.8% we have found significantly higher flux density in Vela's main X-ray peak during radio pulses that arrived early. This excess flux shifts to the 'trough' following the 2nd X-ray peak during radio pulses that arrive later. Our results suggest that the mechanism producing the radio pulses is intimately connected to the mechanism producing X-rays. Current models using resonant absorption of radio emission in the outer magnetosphere as a cause of the X-ray emission are explored as a possible explanation for the correlation.
The history of stellar seismology suggests that observation and theory often take turns advancing our understanding. The recent tripling of the sample of pulsating white dwarfs generated by the Sloan Digital Sky Survey represents a giant leap on the observational side. The time is ripe for a comparable advance on the theoretical side. There are basically two ways we can improve our theoretical understanding of pulsating stars: we can improve the fundamental ingredients of the models, or we can explore the existing models in greater computational detail. For pulsating white dwarfs, much progress has recently been made on both fronts: models now exist that connect the interior structure to its complete evolutionary history, while a method of using parallel computers for global exploration of relatively simple models has also been developed. Future advances in theoretical white dwarf asteroseismology will emerge by combining these two approaches, yielding unprecedented insight into the physics of diffusion, nuclear burning, and mixing.
We have used the infrared mineralogical model derived from the Spitzer IRS observations of the Deep Impact experiment to study the nature of the dust in the debris found around the K0V star HD69830 (Beichman et al. 2005). Using a robust approach to determine the bulk average mineralogical composition of the dust, we show it to be substantially different from that found for comet 9P/Tempel 1 (Lisse et al. 2006) or C/Hale-Bopp 1995 O1 and comet-dominated YSO HD100546 (Lisse et al. 2007). Lacking in carbonaceous and ferrous materials but including small icy grains, the composition of the HD 69830 dust resembles that of a disrupted P or D-type asteroid. The amount of mass responsible for the observed emission is the equivalent of a 30 km radius, 2500 kg m-3 sphere, while the radiative temperature of the dust implies that the bulk of the observed material is at ~1.0 AU from the central source, coincident with the 2:1 and 5:2 mean motion resonances of the outermost of 3 Neptune-sized planets detected by Lovis et al. (2006). In our solar system, P and D-type asteroids are both large and numerous in the outer main belt and near Jupiter (e.g. the Hildas and Trojans) and have undergone major disruptive events to produce debris disk-like structures (c.f. the Karin and Veritas families 5-8 Myrs ago). The short-lived nature of the small and icy dust implies that the disruption occurred within the last year, or that replenishment due to ongoing collisional fragmentation is occurring.
Using a sample of optically-selected quasars from the Sloan Digital Sky Survey, we have determined the radio-loud fraction (RLF) of quasars as a function of redshift and optical luminosity. The sample contains more than 30,000 objects and spans a redshift range of 0<z<5 and a luminosity range of -30<M_i<-22. We use both the radio-to-optical flux ratio (R parameter) and radio luminosity to define radio-loud quasars. After breaking the correlation between redshift and luminosity due to the flux-limited nature of the sample, we find that the RLF of quasars decreases with increasing redshift and decreasing luminosity. The relation can be described in the form of log(RLF/(1-RLF)) = b_0 + b_z log(1+z) + b_M (M_{2500}+26), where M_{2500} is the absolute magnitude at rest-frame 2500A, and b_z, b_M<0. When using R>10 to define radio-loud quasars, we find that b_0=-0.132+/-0.116, b_z=-2.052+/-0.261, and b_M=-0.183+/-0.025. The RLF at z=0.5 declines from 24.3% to 5.6% as luminosity decreases from M_{2500}=-26 to M_{2500}=-22, and the RLF at M_{2500}=-26 declines from 24.3% to 4.1% as redshift increases from 0.5 to 3, suggesting that the RLF is a strong function of both redshift and luminosity. We also examine the impact of flux-related selection effects on the RLF determination using a series of tests, and find that the dependence of the RLF on redshift and luminosity is highly likely to be physical, and the selection effects we considered are not responsible for the dependence.
I review the excursion set theory (EST) of dark matter halo formation and clustering. I recount the Press-Schechter argument for the mass function of bound objects and review the derivation of the Press-Schechter mass function in EST. The EST formalism is powerful and can be applied to numerous problems. I review the EST of halo bias and the properties of void regions. I spend considerable time reviewing halo growth in the EST. This section culminates with descriptions of two Monte Carlo methods for generating halo mass accretion histories. In the final section, I emphasize that the standard EST approach is the result of several simplifying assumptions. Dropping these assumptions can lead to more faithful predictions and a more versatile formalism. One such assumption is the constant height of the barrier for nonlinear collapse. I review implementations of the excursion set approach with arbitrary barrier shapes. An application of this is the now well-known improvement to standard EST that follows from the ellipsoidal-collapse barrier. Additionally, I emphasize that the statement that halo accretion histories are independent of halo environments is a simplifying assumption, rather than a prediction of the theory. I review the method for constructing correlated random walks of the density field in more general cases. I construct a simple toy model with correlated walks and I show that excursion set theory makes a qualitatively simple and general prediction for the relation between halo accretion histories and halo environments: regions of high density preferentially contain late-forming halos and conversely for regions of low density. I conclude with a brief discussion of this prediction in the context of recent numerical studies of the environmental dependence of halo properties. (Abridged)
The determination of the densities of intergalactic photons from the FIR to the UV produced by stellar emission and dust reradiation at various redshifts can provide an independent measure of the star formation history of the universe. High energy gamma-rays can annihilate with FIR-UV photons to produce electron-positron pairs which result in high-end absorption cutoffs in the gamma-ray spectra of extragalactic sources. Future measurements of such absorption in the spectra of extragalactic high energy gamma-ray sources at higher redshifts from detectors such as the (soon to be launched) GLAST space telescope can be used to determine intergalactic photon densities in the distant past, thereby shedding light on the history of star formation and galaxy evolution. (abridged),
Principal Component Analysis (PCA) is a well-known multivariate technique used to decorrelate a set of vectors. PCA has been extensively applied in the past to the classification of stellar and galaxy spectra. Here we apply PCA to the optical spectra of early-type galaxies, with the aim of extracting information about their star formation history. We consider two different data sets: 1) a reduced sample of 30 elliptical galaxies in Hickson compact groups and in the field, and 2) a large volume-limited (z<0.1) sample of ~7,000 galaxies from the Sloan Digital Sky Survey. Even though these data sets are very different, the homogeneity of the populations results in a very similar set of principal components. Furthermore, most of the information (in the sense of variance) is stored into the first few components in both samples. The first component (PC1) can be interpreted as an old population and carries over 99% of the variance. The second component (PC2) is related to young stars and we find a correlation with NUV flux from GALEX. Model fits consistently give younger ages for those galaxies with higher values of PC2.
Since its launch, in late 2004, Swift has been locating gamma-ray bursts (GRBs) at a rate of $\sim$100 per year. The rapid localization and follow-up in many wavelengths is providing new insights and revealing new, and unexpected phenomena, such as bright X-ray flares. Very high energy (VHE) emission ($>$100 GeV) is predicted by several models. Here, we present the results of a search for VHE emission from the most recent GRBs to fall within the Milagro field of view.
The silicate cross section peak near 10um produces emission and absorption features in the spectra of dusty galactic nuclei observed with the Spitzer Space Telescope. Especially in ultraluminous infrared galaxies, the observed absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A foreground screen of obscuration cannot reproduce this observed feature, even at large optical depth. Instead, the deep absorption requires a nuclear source to be deeply embedded in a smooth distribution of material that is both geometrically and optically thick. In contrast, a clumpy medium can produce only shallow absorption or emission, which are characteristic of optically-identified active galactic nuclei. In general, the geometry of the dusty region and the total optical depth, rather than the grain composition or heating spectrum, determine the silicate feature's observable properties. The apparent optical depth calculated from the ratio of line to continuum emission generally fails to accurately measure the true optical depth. The obscuring geometry, not the nature of the embedded source, also determines the far-IR spectral shape.
We present the first results of the VLBA Imaging and Polarimetry Survey (VIPS), a 5 GHz VLBI survey of 1,127 sources with flat radio spectra. Through automated data reduction and imaging routines, we have produced publicly available I, Q, and U images and have detected polarized flux density from 37% of the sources. We have also developed an algorithm to use each source's I image to automatically classify it as a point-like source, a core-jet, a compact symmetric object (CSO) candidate, or a complex source. The mean ratio of the polarized to total 5 GHz flux density for VIPS sources with detected polarized flux density ranges from 1% to 20% with a median value of about 5%. We have also found significant evidence that the directions of the jets in core-jet systems tend to be perpendicular to the electric vector position angles (EVPAs). The data is consistent with a scenario in which ~24% of the polarized core-jets have EVPAs that are anti-aligned with the directions of their jet components and which have a substantial amount of Faraday rotation. In addition to these initial results, plans for future follow-up observations are discussed.
It is quite plausible that color superconductivity occurs in the inner regions of neutron stars. At the same time, it is known that strong magnetic fields exist in the interior of these compact objects. In this paper we discuss some important effects that can occur in the color superconducting core of compact stars due to the presence of the stars' magnetic field. In particular, we consider the modification of the gluon dynamics for a color superconductor with three massless quark flavors in the presence of an external magnetic field. We show that the long-range component of the external magnetic field that penetrates the color-flavor locked phase produces an instability for field values larger than the charged gluons' Meissner mass. As a consequence, the ground state is restructured forming a vortex state characterized by the condensation of charged gluons and the creation of magnetic flux tubes. In the vortex state the magnetic field outside the flux tubes is equal to the applied one, while inside the tubes its strength increases by an amount that depends on the amplitude of the gluon condensate. This paramagnetic behavior of the color superconductor can be relevant for the physics of compact stars.
Close white dwarf binaries make up a wide variety of objects such as double white dwarf binaries, which are possible SN Ia progenitors, cataclysmic variables, super soft sources, or AM CVn stars. The evolution and formation of close white dwarf binaries crucially depends on the rate at which angular momentum is extracted from the binary orbit. The two most important sources of angular momentum loss are the common envelope phase and magnetic braking. Both processes are so far poorly understood. Observational population studies of white dwarf/main sequence binaries provide the potential to significantly progress with this situation and to clearly constrain magnetic braking and the CE-phase. However, the current population of white dwarf/main sequence binaries is highly incomplete and heavily biased towards young systems containing hot white dwarfs. The SDSSII/SEGUE collaboration awarded us with 5 fibers per plate pair in order to fill this gap and to identify the required unbiased sample of old white dwarf/main sequence binaries. The success rate of our selection criteria exceeds 65% and during the first 10 months we have identified 41 new systems, most of them belonging to the missed old population.
In this paper we present results of the statistical analysis of high-latitude HI turbulence in the Milky Way. We have observed HI in the 21 cm line, obtained with the Arecibo L-Band Feed Array (ALFA) receiver at the Arecibo radio telescope. For restoration of velocity statistics we have used the Velocity Coordinate Spectrum (VCS) technique, as at the moment it is the only technique accounting for convergence of lines of sight. In our analysis we have used direct fitting of the VCS model, as its asymptotic regimes are not available with Arecibo resolution and given the restrictions from thermal smoothing of the turbulent line. We have obtained a velocity spectral index 3.90, an injection scale of 105 pc and an HI temperature of 127 K. The spectral index is steeper than a Kolmogorov index and can be interpreted as `shock-type'.
Turbulence is a key element of the dynamics of astrophysical fluids, including those of interstellar medium, clusters of galaxies and circumstellar regions. Turbulent motions induce Doppler shifts of observable emission and absorption lines and this motivates studies of turbulence using precision spectroscopy. We provide high resolution numerical testing of the two promising techniques, namely, Velocity Channel Analysis and Velocity Coordinate Spectrum. We obtain an expression for the shot noise that the discretization of the numerical data entails and successfully test it. We show that numerical resolution required for recovering the underlying turbulent spectrum from observations depend on the spectral index of velocity fluctuations. Thus the low resolution testing may be misleading.
[Abridged] We present new model predictions for 16 Lick absorption line indices from Hdelta through Fe5335, and UBV colors for single stellar populations (SPs) with ages ranging between 1 and 15 Gyr, [Fe/H] ranging from -1.3 to +0.3, and variable abundance ratios. We develop a method to estimate mean ages and abundances of Fe, C, N, Mg, and Ca that explores the sensitivity of the various indices to those parameters. When applied to high-S/N Galactic cluster data, the models match the clusters' elemental abundances and ages with high precision. Analyzing stacked SDSS spectra of early-type galaxies brighter than Lstar, we find mean luminosity-weighted ages of the order of ~ 8 Gyr and iron abundances slightly below solar. Abundance ratios, [X/Fe], are higher than solar, and correlate positively with galaxy luminosity. Nitrogen is the element whose abundance correlates the most strongly with luminosity, which seems to indicate secondary enrichment. This result may impose a lower limit of 50-200 Myr to the time-scale of star formation in early-type galaxies. Unlike in the case of clusters, in galaxies bluer Balmer lines yield younger ages than Hbeta. This age discrepancy is stronger for lower luminosity galaxies. We examine four scenarios to explain this trend. The most likely is the presence of small amounts of a young/intermediate-age SP component. Two-component models provide a better match to the data when the mass fraction of the young component is a few %. This result implies that star formation has been extended in early-type galaxies, and more so in less massive galaxies, lending support to the ``downsizing'' scenario. It also implies that SP synthesis models are capable of constraining not only the mean ages of SPs in galaxies, but also their age spread.
We analyze a technique of obtaining turbulence power spectrum using spectral line data along the velocity coordinate, which we refer to as Velocity Coordinate Spectrum (VCS). We formalize geometrical aspects of observation through a single factor, "geometric term". We find that all variety of particular observational configurations can be described using correspondent variants of this term, which we explicitly calculate. This allows us to obtain asymptotics for both parallel lines of sight and crossing lines of sight. The latter case is especially important for studies of turbulence within diffuse ISM in Milky Way. For verification of our results, we use direct calculation of VCS spectra, while the numerical simulations are presented in a companion paper.
We present theoretical predictions for photometric and spectroscopic signatures of rings around transiting extrasolar planets. On the basis of a general formulation for the transiting signature in the stellar light curve and the velocity anomaly due to the Rossiter effect, we compute the expected signals analytically for a face-on ring system, and numerically for more general configuration. In particular we consider the detectability of planetary rings around HD209458b and Saturn, and find that they are indeed marginally detectable even with the current observational accuracy as long as sufficiently sampled data points are available. Definitely the detection of rings around extrasolar transiting planets should be one of the major scientific goals of the up-coming space missions.
We report our observations on 4 AGNs viz, Mkn421, Mkn501, 1ES1426+428 and ON231 belonging to a sub-class called blazars. The observations were carried out using the Pachmarhi Array of Cherenkov Telescopes and span about 6 years period from 2000 to 2005. We discuss our methods of analysis adopted to extract the gamma ray signal from cosmic ray background. We present our results on the emission of TeV gamma rays from these objects. Also, we report on the status of the new high altitude experiment, a 7 telescope array at Hanle in the Himalayas at an altitude of about 4200 m above mean sea level for detecting celestial gamma rays.
Turbulence in an incompressible fluid with and without a magnetic field as well as moderately compressible MHD turbulence are compared. For the magnetohydrodynamic (MHD) models the probability distribution functions of the velocity components perpendicular to the external magnetic field are like the incompressible hydrodynamic (HD) model while those parallel to the field have a smaller range of velocities. The probability distribution functions of the transverse velocity increments for the MHD models decline slower than the incompressible HD model. The similarity of incompressible HD and both incompressible and compressible MHD turbulence persists over high order longitudinal structure function scaling exponents measured in the global reference frame as well as for motions perpendicular to the local mean field. In these two frames the longitudinal scaling exponents of both MHD models seem to follow theoretical incompressible HD dissipation structure predictions while the transverse scaling exponents of the incompressible MHD model seem to follow the predictions for incompressible MHD. In the local magnetic system the motions parallel to the local mean field for both MHD models are different from incompressible HD motions. The fields of the MHD simulations are decomposed into Alfvenic, fast, and slow modes and stuided separately. (abridged)
I study the behaviour of the maximum rms fractional amplitude, $r_{\rm max}$ and the maximum coherence, $Q_{\rm max}$, of the kilohertz quasi-periodic oscillations (kHz QPOs) in a dozen low-mass X-ray binaries. I find that: (i) The maximum rms amplitudes of the lower and the upper kHz QPO, $r^{\ell}_{\rm max}$ and $r^{\rm u}_{\rm max}$, respectively, decrease more or less exponentially with increasing luminosity of the source; (ii) the maximum coherence of the lower kHz QPO, $Q^{\ell}_{\rm max}$, first increases and then decreases exponentially with luminosity; (iii) the maximum coherence of the upper kHz QPO, $Q^{\rm u}_{\rm max}$, is more or less independent of luminosity; and (iv) $r_{\rm max}$ and $Q_{\rm max}$ show the opposite behaviour with hardness of the source, consistent with the fact that there is a general anticorrelation between luminosity and spectral hardness in these sources. Both $r_{\rm max}$ and $Q_{\rm max}$ in the sample of sources, and the rms amplitude and coherence of the kHz QPOs in individual sources show a similar behaviour with hardness. This similarity argues against the interpretation that the drop of coherence and rms amplitude of the lower kHz QPO at high QPO frequencies in individual sources is a signature of the innermost stable circular orbit around a neutron star.
We present an analysis of all SAX observations of NGC 4151. This source was observed 5 times from 1996 to 2001 with durations ranging from a day to four days. The intrinsic continuum, is absorbed at low energies by a complex system: a cold patchy absorber plus a warm uniform screen photoionized by the central continuum. We find that this dual absorber is the main driver of the observed variability, up to a factor of eight, at 3 keV. In particular the covering fraction of the cold absorber changes on time scales of the order of a day, supporting its association with the Broad Line Region. The column density of the warm gas varies on a longer time scale (months to year). Some of the small amplitude spectral variability above 10 keV can be explained with an intrinsic variation (\Delta\Gamma~0.2).The flux below 1 keV remains constant confirming an extended origin. Its spectrum is reproduced by a combination of a thermal component and a power law with the same slope as the intrinsic continuum but with an intensity a few per cent. A Compton reflection component is significantly detected in 1996 (averaged value of \Omega/2\pi ~0.4), with intensity decreasing on time scale of year, and it desappears in 2000 and 2001. The long time scale of variations argues for an association with an optically thick torus at a distance of few light years. An iron line was detected in all spectra. Its energy is consistent with fluorescence by cold iron. We find that the line is variable. Its behaviour is reproduced by a variable component proportional to the level of the reflection flux plus a constant component. The flux of the latter is consistent with the extended line emission observed by Chandra. We conclude that the first component is likely arising from the torus and the second is produced in the extended Narrow Line Region.
Advances in instrumentation have made it possible to study sunspots with unprecedented detail. New capabilities include imaging observations at a resolution of 0.1" (70 km on the sun), spectroscopy at ~0.2", and simultaneous spectropolarimetry in visible and infrared lines at resolutions well below 1". In spite of these advances, we still have not identified the building blocks of the penumbra and the mechanism responsible for the Evershed flow. Three different models have been proposed to explain the corpus of observations gathered over the years. The strengths and limitations of these models are reviewed in this contribution.
The Galactic Center was observed at 8.6 mum in order to detect a mid-infrared (MIR) counterpart of Sgr A*, parallel to NIR observations. The goal was to set constraints on possible emission mechanisms.Imaging data were acquired with the adaptive optics assisted NIR instrument NACO and the MIR instrument VISIR at the ESO VLT. We present MIR imaging data of an unprecedented quality in terms of spatial resolution and sensitivity. An extended ridge of emission is found to be present in the immediate vicinity of Sgr A* and thus renders any detection of a point source difficult. No MIR point source related to Sgr A* was detected during the observations. We derive a tight upper limit of 22+-4 mJy (dereddened) on any possible point source present during the observations in the night of 4/5 June 2006. The absence of a flare in simultaneous observations at 2.2 mum and the low limits on any possible variability in the MIR suggest strongly that Sgr A* was in a quasi-quiescent state during this night. During the night from 5 to 6 June 2006, Sgr A* was found to be variable on a low level at 3.8 mum. No point source at 8.6 mum was detected during the simultaneous MIR observations. Due to the poorer atmospheric conditions a higher upper limit of 60+-30mJy was found for Sgr A* at 8.6 mum during the second night. The observations are consistent with theoretical predictions. If the published models are correct, the observations demonstrate successfully that a 8.6 mum counterpart of Sgr A* can be easily detected in its flaring state. Spectral indices derived from simultaneous observations of flaring emission from Sgr A* at NIR and MIR wavelengths will enable us to distinguish between different kinds of flare models.
In this paper, we try to find any dependence of the Galactic structural parameters with the Galactic longitude for either the thin disc or the thick disc of the Milky Way, that would indicate to possible inhomogeneities or asymmetries in those Galactic components. Galactic model parameters obtained using a set of 36 high-latitude fields with 2MASS photometry for the different fields show that, effectively, they are Galactic longitude-dependent. The thick disc scaleheight changes from 800 pc at 150 deg < l < 230 deg to 1050 pc at |l| < 30 deg. A plausible explanation for this finding might be the effect of the flare in this Galactic component that changes the scaleheight with Galactocentric distance
The XAA1.2 is a custom ASIC chip for silicon microstrip detectors adapted by Ideas for the SuperAGILE instrument on board the AGILE space mission. The chip is equipped with 128 input channels, each one containing a charge preamplifier, shaper, peak detector and stretcher. The most important features of the ASIC are the extended linearity, low noise and low power consumption. The XAA1.2 underwent extensive laboratory testing in order to study its commandability and functionality and evaluate its scientific performances. In this paper we describe the XAA1.2 features, report the laboratory measurements and discuss the results emphasizing the scientific performances in the context of the SuperAGILE front-end electronics.
We have studied optical spectra of the symbiotic star Z And, obtained during its latest outburst started in April 2006, with the aim of finding changes in the spectrum yielding clues to the nature of the hot component and its outbursts. The spectroscopic observations of Z And have been made using the 1.5-meter telescope at the Tartu Observatory, Estonia, and processed in a standard way. We have found high velocity satellites to the hydrogen Balmer emission lines. Starting from July 30, 2006, weak additional emission components at velocities of about +/-1150 km/s were detected. Their appearance near the outburst maximum and similarity to the emission features in another symbiotic star Hen 3-1341 imply fast collimated outflows from the hot component of Z And. This finding is consistent with the earlier results by several authors that symbiotic stars can emit bipolar jets at certain stages of their outbursts. A significant decrease in the temperature of the hot component in initial stages of the outburst was detected by the disappearance of the high excitation emission line from the spectrum.
A short overview is presented of several topics concerning the evolution of the Milky Way (MW) in a cosmological context. In particular, the metallicity distribution of the MW halo is derived analytically and the halo metallicity and abundance patterns are compared to those of Local Group galaxies. The inside-out formation of the MW disk is supported by the observed evolution of the abundance gradients, while their magnitude suggests that the role of the Galactic bar has been negligible. Finally, the empirical foundations (age-metallicity relation and metallicity distribution) of the evolution of the solar neighborhood, which is the best studied galactic sub-system, have been seriously questioned recently.
(Abridged) We present results of several X-ray observations of the X-ray binary 4U 1954+31 performed with the satellites BeppoSAX, EXOSAT, ROSAT, RXTE, and Swift. We also studied the RXTE ASM data over a period of more than 10 years. Light curves of all observations show an erratic behaviour with sudden increases in the source emission on timescales variable from hundreds to thousands of seconds. There are no indications of changes in the source spectral hardness, with the possible exception of the RXTE pointed observation. Timing analysis does not reveal the presence of coherent pulsations or periodicities either in the pointed observations in the range from 2 ms to 2000 s or in the long-term RXTE ASM light curve on timescales from days to years. The 0.2-150 keV spectrum, afforded with BeppoSAX, is the widest for this source available up to now in terms of spectral coverage and is well described by a model consisting of a lower-energy thermal component (hot diffuse gas) plus a higher-energy (Comptonization) emission, with the latter modified by a partially-covering cold absorber plus a warm (ionized) absorber. A blackbody modelization of our BeppoSAX low-energy data is instead ruled out. The presence of a complex absorber local to the source is also supported by the 0.1-2 keV ROSAT spectrum. RXTE, EXOSAT and Swift X-ray spectroscopy is consistent with the above results, but indicates variations in the density and the ionization of the local absorber. A 6.5 keV emission line is possibly detected in the BeppoSAX and RXTE spectra. All this information suggests that the scenario which better describes 4U 1954+31 consists of a binary system in which a neutron star orbits in a highly inhomogeneus medium from a stellar wind coming from its optical companion, an M-type giant star.
We consider the linear stability of dissipative MHD Taylor-Couette flow with
imposed toroidal magnetic fields. The inner and outer cylinders can be either
insulating or conducting; the inner one rotates, the outer one is stationary.
The magnetic Prandtl number can be as small as 10<sup>-5</sup>, approaching
realistic liquid-metal values. The magnetic field destabilizes the flow, except
for radial profiles of B<sub>\phi</sub>(R) close to the current-free solution.
The profile with B<sub>in</sub>=B<sub>out</sub> (the most uniform field) is
considered in detail. For weak fields the TC-flow is <I>stabilized</I>, until
for moderately strong fields the m=1 azimuthal mode dramatically destabilizes
the flow again. There is thus a maximum value for the critical Reynolds number.
For sufficiently strong fields (as measured by the Hartmann number) the
toroidal field is always unstable, even for Re=0.
The electric currents needed to generate the required toroidal fields in
laboratory experiments are a few kA if liquid sodium is used, somewhat more if
gallium is used. Weaker currents are needed for wider gaps, so a wide-gap
apparatus could succeed even with gallium. The critical Reynolds numbers are
only somewhat larger than the nonmagnetic values, so such an experiment would
require only modest rotation rates.
The CERN Axion Solar Telescope (CAST) is searching for solar axions using the 9.0 T strong and 9.26 m long transverse magnetic field of a twin aperture LHC test magnet, where axions could be converted into X-rays via reverse Primakoff process. Here we explore the potential of CAST to search for 14.4 keV axions that could be emitted from the Sun in M1 nuclear transition between the first, thermally excited state, and the ground state of 57Fe nuclide. Calculations of the expected signals, with respect to the axion-photon coupling, axion-nucleon coupling and axion mass, are presented in comparison with the experimental sensitivity.
(Abridged) We aim to enlarge the number of known hot corinos and carry out a first comparative study with hot cores. The ultimate goal is to understand whether complex organic molecules form in the gas phase or on grain surfaces, and what the possible key parameters are. We observed millimeter rotational transitions of HCOOH, HCOOCH3, CH3OCH3, CH3CN, and C2H5CN in a sample of low-mass protostars with the IRAM-30m. Using the rotational diagram method coupled with the information about the sources' structure, we calculate the abundances of the observed molecules. To interpret these abundances, we review the proposed formation processes of the above molecules. We report the detection of HCOOCH3 and/or CH3CN towards NGC1333-IRAS4B and NGC1333-IRAS2A. We find that abundance ratios of O-bearing molecules to methanol or formaldehyde in hot corinos are comparable and about unity, and are relatively (depending on how the ratios are determined) higher than those in hot cores and in Galactic center clouds. So far, complex organic molecules were detected in all the hot corinos where they were searched for, suggesting that it is a common phase for low-mass protostars. While some evidence points to grain-surface synthesis (either in the cold or warm-up phase) of these molecules (in particular for HCOOH and HCOOCH3), the present data do not allow us to disregard gas-phase formation. More observational, laboratory, and theoretical studies are required to improve our understanding of hot corinos.
The most efficient energy sources known in the Universe are accretion disks. Those around black holes convert 5 -- 40 per cent of rest-mass energy to radiation. Like water circling a drain, inflowing mass must lose angular momentum, presumably by vigorous turbulence in disks, which are essentially inviscid. The origin of the turbulence is unclear. Hot disks of electrically conducting plasma can become turbulent by way of the linear magnetorotational instability. Cool disks, such as the planet-forming disks of protostars, may be too poorly ionized for the magnetorotational instability to occur, hence essentially unmagnetized and linearly stable. Nonlinear hydrodynamic instability often occurs in linearly stable flows (for example, pipe flows) at sufficiently large Reynolds numbers. Although planet-forming disks have extreme Reynolds numbers, Keplerian rotation enhances their linear hydrodynamic stability, so the question of whether they can be turbulent and thereby transport angular momentum effectively is controversial. Here we report a laboratory experiment, demonstrating that non-magnetic quasi-Keplerian flows at Reynolds numbers up to millions are essentially steady. Scaled to accretion disks, rates of angular momentum transport lie far below astrophysical requirements. By ruling out purely hydrodynamic turbulence, our results indirectly support the magnetorotational instability as the likely cause of turbulence, even in cool disks.
We review the results of our analysis of the OGLE LMC eclipsing binaries
(Mazeh, Tamuz & North 2006), using EBAS -- Eclipsing Binary Automated Solver,
an automated algorithm to fit lightcurves of eclipsing binaries (Tamuz, Mazeh &
North 2006).
After being corrected for observational selection effects, the set of
detected eclipsing binaries yielded the period distribution and the frequency
of all LMC short-period binaries, and not just the eclipsing systems.
Somewhat surprisingly, the period distribution is consistent with a flat
distribution in log P between 2 and 10 days. The total number of binaries with
periods shorter than 10 days in the LMC was estimated to be about 5000. This
figure led us to suggest that (0.7 +- 0.4)% of the main-sequence A- and B-type
stars are found in binaries with periods shorter than 10 days. This frequency
is substantially smaller than the fraction of binaries found by small Galactic
radial-velocity surveys of B stars.
We review the state of the art for measuring the X-ray polarization of neutron stars. We discuss how valuable precision measurements of the degree and position angle of polarization as a function of energy and, where relevant, of pulse phase, would provide deeper insight into the details of the emission mechanisms. We then review the current state of instrumentation and its potential for obtaining relevant data. Finally, we conclude our discussion with some opinions as to future directions.
We have studied a sample of 28 periodically variable B-type supergiants
selected from the HIPPARCOS mission and 12 comparison stars covering the whole
B-type spectral range. Our goal is to test if their variability is compatible
with opacity-driven non-radial oscillations.
We have used the NLTE atmosphere code FASTWIND to derive the atmospheric and
wind parameters of the complete sample through line profile fitting. We applied
the method to selected H, He and Si line profiles, measured with the high
resolution CES spectrograph attached to the ESO CAT telescope in La Silla,
Chile.
From the location of the stars in the (log Teff, log g) diagram, we suggest
that variability of our sample supergiants is indeed due to the gravity modes
resulting from the opacity mechanism. We find nine of the comparison stars to
be periodically variable as well, and suggest them to be new alpha Cyg
variables. We find marginal evidence of a correlation between the amplitude of
the photometric variability and the wind density. We investigate the Wind
Momentum Luminosity Relation for the whole range of B spectral type
supergiants, and find that the later types (> B5) perfectly follow the relation
for A supergiants. Additionally, we provide a new spectral type - Teff
calibration for B supergiants.
Our results imply the possibility to probe internal structure models of
massive stars of spectral type B through seismic tuning of gravity modes.
We estimated the Galactic model parameters from the SDSS data reduced for two fields in the anti-centre direction of the Galaxy, $l= 180^{o}$, and symmetric relative to the Galactic plane, $b=+45^{o}$ and $b=-45^{o}$. The large size of each field, 60 deg$^{2}$, and the faint limiting apparent magnitude, g=22, give us the chance to determine reliable parameters for three components, thin and thick discs and halo, in the north and south hemispheres of the Galaxy, except the scalelengths for two discs which are adopted from Juri\'c et al. (2005). Metallicities were evaluated by a recent calibration for SDSS, and absolute magnitudes of stars with $4<M(g)\leq8$ were derived as a function of (g-r) colour and metallicity. Conspicuous differences could not be detected between the corresponding Galactic model parameters for the thin disc of north and south fields, and our results are consistent with the ones in the literature. The same case is valid for the halo, especially the axis ratios for two fields are exactly equal, $\kappa=0.45$. The metallicity distribution for unevolved G type stars with $5<M(g)\leq6$ shows three substructures relative to the distance from the Galactic plane: for $z^{*}<3$ kpc, the metallicity gradient for two fields is $d[M/H]/dz \sim -0.20(\pm0.02)$ dex kpc$^{-1}$, consistent with the formation scenario of the thin disc. For $5<z^{*}<10$ kpc, $d[M/H]/dz \sim -0.03(\pm0.001)$ dex kpc$^{-1}$ for two fields, confirming that the metallicity gradient for the halo component is close to zero. When we combine these substructures, however, we find a smooth metallicity gradient for two fields, $-0.30(\pm0.04)$ dex kpc$^{-1}$.
We report the discovery using Hubble Space Telescope imaging and Keck spectroscopy of a very bright, highly magnified (~30*) Lyman Break Galaxy (LBG) at z=3.07 in the field of the massive z=0.33 cluster MACSJ2135.2-0102. The system comprises two high-surface brightness arcs with a maximum extent of 3", bracketing a central object which we identify as a massive early-type galaxy at z=0.73. We construct a lens model which reproduces the main features of the system using a combination of a galaxy-scale lens and the foreground cluster. We show that the morphological, spectral and photometric properties of the arcs are consistent with them arising from the lensing of a single ~L* LBG. The most important feature of this system is that the lensing magnification results in an apparent magnitude of r=20.3, making this one of the brightest LBGs known. Such a high magnification provides the opportunity of obtaining very high signal to noise (and potentially spatially resolved) spectroscopy of a high redshift galaxy to study its physical properties. We present initial imaging and spectroscopy demonstrating the basic properties of the system and discuss the opportunities for future observations.
Context: Accretion onto galactic and supermassive black holes occurs in
different modes, which are documented in hard and soft spectral states,
commonly attributed to an advection-dominated flow (ADAF) inside a truncated
disk and standard disk accretion, respectively.
At the times of spectral transition an intermediate state is observed, for
which the accretion flow pattern is still unclear.
Aims: We analyze the geometry of the accretion flow when the mass flow rate
in the disk decreases (soft/hard transition) and evaporation of gas into the
coronal flow leads to disk truncation.
Methods: We evaluate the physics of an advection-dominated flow affected by
thermal conduction to a cool accretion disk underneath.
We find re-condensation of gas from the ADAF into the underlying inner disk
at distances from the black hole and at rates, that depend on the properties of
the hot ADAF and vary with the mass accretion rate. This sustains an inner disk
for longer than a viscous decay time after the spectral transition occurred, in
accordance with the spectra that indicate cool gas in the neighborhood of the
accreting black hole. The model allows us to understand why Cyg X-1 does not
show hysteresis in the spectral state transition luminosity that is commonly
observed for X-ray transient sources.
Conclusions: Our results shed new light on the complex mass flow pattern
during spectral state transition.
Active prominences exhibit plasma motions, resulting in difficulties with the interpretation of spectroscopic observations. These solar features being strongly influenced by the radiation coming from the solar disk, Doppler dimming or brightening effects may arise, depending on which lines are observed and on the velocity of the plasma. Interlocking between the different atomic energy levels and non local thermodynamic equilibrium lead to non-trivial spectral line profiles, and this calls for complex numerical modelling of the radiative transfer in order to understand the observations. We present such a tool, which solves the radiative transfer and statistical equilibrium for H, He I, He II, and Ca II, in moving prominences where radial plasma motions are taking place. It is found that for isothermal, isobaric prominence models, the He II resonance lines are very sensitive to the Doppler effect and show a strong Doppler dimming. The Ca II lines are not very sensitive to the Doppler effect for the prominence models considered here. We illustrate how the code makes it possible to retrieve the plasma thermodynamic parameters by comparing computed and observed line profiles of hydrogen and helium resonance lines in a quiescent prominence.
A breakthrough in the sensitivity level of the hard X-/gamma-ray telescopes, which today are based on detectors that view the sky through (or not) coded masks, is expected when focusing optics will be available also in this energy range. Focusing techniques are now in an advanced stage of development. To date the most efficient technique to focus hard X-rays with energies above 100 keV appears to be the Bragg diffraction from crystals in transmission configuration (Laue lenses). Crystals with mosaic structure appear to be the most suitable to build a Laue lens with a broad passband, even though other alternative structures are being investigated. The goal of our project is the development of a broad band focusing telescope based on gamma-ray lenses for the study of the continuum emission of celestial sources from 60 keV up to >600 keV. We will report details of our project, its development status and results of our assessment study of a lens configuration for the European Gamma Ray Imager (GRI) mission now under study for the ESA plan "Cosmic Vision 2015-2025".
Sunspots are locations on the Sun where unique atmospheric conditions prevail. In particular, the very low temperatures found above sunspots allow the emission of H_2 lines. In this study we are interested in the radiation emitted by sunspots in the O VI lines at 1031.96 A and 1037.60 A. We use SOHO/SUMER observations of a sunspot performed in March 1999 and investigate the interaction between the O VI lines and a H_2 line at 1031.87 A found in the Werner band. The unique features of sunspots atmospheres may very well have important implications regarding the illumination of coronal O+5 ions in the low corona, affecting our interpretation of Doppler dimming diagnostics.
This paper sets out to measure the timescale of quasar variability with a view to new understanding of the size of accretion discs in active galactic nuclei. Previous attempts to measure such timescales have been based on sparsely sampled data covering small ranges of time. Here we combine data from two large scale monitoring programmes to obtain Fourier power spectra of light curves covering nearly three orders of magnitude in frequency in blue and red passbands. If the variations are interpreted as due to gravitational microlensing, then timescale measurements in the observer's frame imply a minimum mass for the microlensing bodies of around $0.4 M_{\odot}$. On the assumption that the variations are intrinsic to the quasars, a correction must be made for time dilation. In this case the power spectrum shows a break corresponding to a timescale of about 11 years. This timescale is used to measure the size of the accretion disc, which is found to be about $10^{-2}$ pc or 10 light days, in agreement with limits set by self-gravitation and coincident with the broad line region of the active galactic nucleus. It is suggested that the broad line region may be associated with the break up of the outer part of the accretion disc.
The observational record of turbulence within the molecular gas phase of the interstellar medium is summarized. We briefly review the analysis methods used to recover the velocity structure function from spectroscopic imaging and the application of these tools on sets of cloud data. These studies identify a near-invariant velocity structure function that is independent of local the environment and star formation activity. Such universality accounts for the cloud-to-cloud scaling law between the global line-width and size of molecular clouds found by Larson (1981) and constrains the degree to which supersonic turbulence can regulate star formation. In addition, the evidence for large scale driving sources necessary to sustain supersonic flows is summarized.
In this paper we report on the third soft gamma-ray source catalog obtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRAL satellite. The scientific dataset is based on more than 40 Ms of high quality observations performed during the first three and a half years of Core Program and public IBIS/ISGRI observations. Compared to previous IBIS/ISGRI surveys, this catalog includes a substantially increased coverage of extragalactic fields, and comprises more than 400 high-energy sources detected in the energy range 18-100 keV, including both transients and faint persistent objects which can only be revealed with longer exposure times.
We present MOND analysis for several of the lowest mass disc galaxies currently amenable to such analysis--with (baryonic) masses below 4x10^8 solar masses. The agreement is good, extending the validity of MOND and its predicted mass velocity relation, to such low masses.
We have designed and constructed a ``dispersed Fourier Transform Spectrometer'' (dFTS), consisting of a conventional FTS followed by a grating spectrometer. By combining these two devices, we negate a substantial fraction of the sensitivity disadvantage of a conventional FTS for high resolution, broadband, optical spectroscopy, while preserving many of the advantages inherent to interferometric spectrometers. In addition, we have implemented a simple and inexpensive laser metrology system, which enables very precise calibration of the interferometer wavelength scale. The fusion of interferometric and dispersive technologies with a laser metrology system yields an instrument well-suited to stellar spectroscopy, velocimetry, and extrasolar planet detection, which is competitive with existing high-resolution, high accuracy stellar spectrometers. In this paper, we describe the design of our prototype dFTS, explain the algorithm we use to efficiently reconstruct a broadband spectrum from a sequence of narrowband interferograms, and present initial observations and resulting velocimetry of stellar targets.
We present X-ray and weak lensing observations of the merging cluster system 1E0657-556. Due to the recently collision of a merging subcluster with the main cluster, the X-ray plasma has been displaced from the cluster galaxies in both components. The weak lensing data shows that the lensing surface potential is in spatial agreement with the galaxies (~10% of the observed baryons) and not with the X-ray plasma (~90% of the observed baryons). We argue that this shows that regardless of the form of the gravitational force law at these large distances and low accelerations, these observations require that the majority of the mass of the system be some form of unseen matter.
We implement the theory of resonant scattering in the context of Cosmic Microwave Background (CMB) polarisation anisotropies. We compute the changes in the E-mode polarisation (EE) and Temperature E-mode (TE) CMB power spectra introduced by the scattering on a resonant transition with a given optical depth $\tau_X$ and polarisation coefficient $E_1$. The latter parameter, accounting for how anisotropic the scattering is, depends on the exchange of angular momentum in the transition and enables, {\it a priori}, to distinguish among different possible resonances. We use this formalism in two different scenarios: cosmological recombination and cosmological reionisation. In the context of cosmological recombination, we compute predictions in frequency and multipole space for the change in the TE and EE power spectra introduced by scattering on the $H_{\alpha}$ and $P_{\alpha}$ lines of Hydrogen. This constitutes a fundamental test for the standard model of recombination, and the sensitivity it requires is comparable to that needed in measuring the primordial CMB B-mode polarisation component. In the context of reionisation, we study the scattering off metals and ions produced by the first stars, and find that polarisation anisotropies, apart from providing a consistency test for intensity measurements, give some insight on how reionisation evolved: since they keep memory of how anisotropic the line scattering is, they should be able to discern the OI 63.2$\mu$m transition from other possible transitions associated to OIII, NII, NIII, etc. The amplitude of these signals are, however, between 10 to 100 times below the (already challenging) level of CMB B-mode polarisation anisotropies.
Extremely low mass white dwarfs are very rare objects likely formed in compact binary systems. We present MMT optical spectroscopy of 42 low mass white dwarf candidates serendipitously discovered in a survey for hypervelocity B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \pm 72 K and log g = 5.48 \pm 0.03; with an estimated mass of 0.17 M_sun, it is the lowest gravity/mass white dwarf currently known. However, 40 of the low mass candidates are normal DA white dwarfs with apparently inaccurate SDSS g magnitudes. We revisit the identification of low mass white dwarf candidates previously found in the SDSS, and conclude that four objects have M < 0.2 M_sun. None of these white dwarfs show excess emission from a binary companion, and radial velocity searches will be necessary to constrain the nature of the unseen companions.
A numerical method for integrating the equations describing a dynamically coupled system made of a fluid and cosmic-rays is developed. In smooth flows the effect of CR pressure is accounted for by modification of the characteristic equations and the energy exchange between cosmic-rays and the fluid, due to diffusive processes in configuration and momentum space, is modeled with a flux conserving method. Provided the shock acceleration efficiency as a function of the upstream conditions and shock Mach number, we show that the Riemann solver can be modified to take into account the cosmic-ray mediation without having to resolve the cosmic-ray induced substructure. Shocks are advanced with Glimm's method which preserves without smearing their discontinuous character, allowing to maintain self-consistency in the shock solutions. In smooth flows either Glimm's or a higher order Godunov's method can be applied, with the latter producing better results when approximations are introduced in the Riemann solver.
Time series of high-resolution images of the complex active region NOAA 10786 are studied. The observations were performed in G-band (430.5 nm) and in the nearby continuum (463.3 nm), on July 9, 2005 at the Swedish 1-meter Solar Telecope (SST) in La Palma. Granular proper motions in the surroundings of the sunspots have been quantified. A large-scale radial outflow in the velocity range 0.3 - 1 km s^[-1] has been measured around the sunspots by using local correlation tracking techniques. However, this outflow is not found in those regions around the sunspots with no penumbral structure. This result evidences an association between penumbrae and the existence of strong horizontal outflows (the moat) in sunspots.
We present an analysis of NICMOS photometry and low-resolution grism spectroscopy of low-mass stars and sub-stellar objects in the young star-forming region NGC 1333. Our goal is to constrain the ratio of low-mass stars to sub- stellar objects down to 20 Mjup in the cluster as well as constrain the cluster IMF down to 30 Mjup in combination with a previous survey of NGC 1333 by Wilking et al. Our survey covers 4 fields of 51.2" x 51.2", centered on brown dwarf candidates previously identified in Wilking et al. We extend previous work based on the use of a water vapor index for spectral typing to wavelengths accessible with NICMOS on the HST. Spectral types were derived for the 14 brightest objects in our fields, ranging from <M0 - M8, which at the age of the cluster (0.3 Myr) corresponds to a range in mass of >0.25 - 0.02 Msun. In addition to the spectra, we present an analysis of the color-magnitude diagram using pre-main sequence evolutionary models of D'Antona & Mazzitelli. Using an extinction-limited sample, we derive the ratio of low-mass stars to brown dwarfs. Comparisons of the observed ratio to that expected from the field IMF of Chabrier indicate that the two results are consistent. We combine our data with that of Wilking et al. to compute the ratio of intermediate-mass stars (0.1 - 1.0 Msun) to low-mass objects (0.03 - 0.1 Msun) in the cluster. We also report the discovery of a faint companion to the previously confirmed brown dwarf ASR 28, as well as a possible outflow surrounding ASR 16. If the faint companion is confirmed as a cluster member, it would have a mass of ~ 5 Mjup (mass ratio 0.15) at a projected distance of 350 AU, similar to 2MASS 1207-3923 B.
A search for optical line emission from the two-photon decay of relic axions was conducted in the galaxy clusters Abell 2667 and 2390, using spectra from the Visible Multi-Object Spectrograph (VIMOS) Integral Field Unit (IFU) at the Very Large Telescope (VLT). New upper limits to the two-photon coupling of the axion are derived, and are at least a factor of 3 more stringent than previous upper limits in this mass window. The improvement follows from larger collecting area, integration time, and spatial resolution, as well as from improvements in signal to noise and sky subtraction made possible by strong-lensing mass models of these clusters. The new limits either require that the two-photon coupling of the axion be extremely weak or that the axion mass window between 4.5 eV and 7.7 eV be closed.
Neutrino astronomy was initiated primarily to search for TeV to PeV neutrinos from Active Galactic Nuclei, and the optical Cherenkov technique is well suited for this energy range. Interest has grown recently in detecting EeV neutrinos, particularly the ``cosmogenic'' neutrinos produced during propagation of ultra-high-energy cosmic rays (UHECR) through the microwave background radiation. These neutrinos could be a powerful tool both to resolve the mystery of the UHECR sources and to test fundamental physics at the $\sim$100 TeV scale. The optical technique is not cost effective at these energies and newer techniques such as radio and acoustic detection are necessary. Accelerator experiments have confirmed the production of both types of signals from high-energy showers in various media, and quantitative measurements have confirmed theoretical descriptions of the signal strength, frequency content and pulse shape. While radio experiments have set the strongest limits so far, the acoustic method could contribute with an entirely independent signal production and detection mechanism and may be more effective at the highest energies. Efforts are underway to develop the acoustic method in various media around the world, with arrays operating in ocean water at the Bahamas, the UK, and the Mediterranean Sea; detectors prepared for deployment in the South Pole ice in the next year; and ideas for future acoustic detectors in salt domes and on Antarctica's Ross Ice Shelf. Regardless of which method is individually most sensitive, the best configuration may be to co-deploy arrays to combine the techniques and seek coincident detection of individual neutrino events.
A test of Newton's law of gravity in the low acceleration regime using globular clusters is presented. New results for the core collapsed globular cluster NGC 7099 are given. The run of the gravitational potential as a function of distance is probed studying the velocity dispersion profile of the cluster, as derived from a set of 125 radial velocities with accuracy better than 1 km/s. The velocity dispersion profile is traced up to ~18 pc from the cluster center. The dispersion is found to be maximal at the center, then decrease until 10+-2 pc from the center, well inside the cluster tidal radius of 42 pc. After that the dispersion remains constant with average value 2.2+-0.3 km/s. Assuming for NGC 7099 a total V mag of M(V)=-7.43 mags and mass-to-light ratio M/L=1, the acceleration at 10 pc from the center is 1.1e-8 cm/s/s. Thus, the flattening of the velocity dispersion profile occurs for a value of the internal acceleration of gravity fully consistent with a_0=1.2e-8 cm/s/s observed in galaxies. This new result for NGC 7099 brings to 4 the clusters with velocity dispersion profile probing acceleration below a_0. All four have been found to have a flat dispersion profile at large radii where the acceleration is below a_0, mimicking qualitatively and quantitatively elliptical galaxies. Whether this indicates a failure of Newtonian dynamics in the low acceleration limit or some more conventional dynamical effect (e.g., tidal heating) is still unclear. However, the similarities emerging between very different globular clusters, as well as between globular clusters and elliptical galaxies seem to favor the first of these two possibilities.
High-resolution imaging has revealed an unusually high binary fraction amongst objects spanning the transition between the L dwarf and T dwarf spectral classes. In an attempt to reproduce and unravel the origins of this apparent binary excess, I present a series of Monte Carlo mass function and multiplicity simulations of field brown dwarfs in the vicinity of the Sun. These simulations are based on the solar metallicity brown dwarf evolutionary models and incorporate empirical luminosity and absolute magnitude scales, measured multiplicity statistics and observed spectral templates in the construction and classification of composite binary spectra. In addition to providing predictions on the number and surface density distributions of L and T dwarfs for volume-limited and magnitude-limited samples, these simulations successfully reproduce the observed binary fraction distribution assuming an intrinsic (resolved) binary fraction of 11(+6)(-3)% (95% confidence interval), consistent with prior determinations. However, the true binary fraction may be as high as 40% if, as suggested by Liu et al., a significant fraction of L/T transition objects (~66%) are tightly-bound, unresolved multiples. The simulations presented here demonstrate that the binary excess amongst L/T transition objects arises primarily from the flattening of the luminosity scale over these spectral types and is not inherently the result of selection effects incurred in current magnitude-limited imaging samples. Indeed, the existence of a binary excess can be seen as further evidence that brown dwarfs traverse the L/T transition rapidly, possibly driven by a nonequilibrium submergence of photospheric condensates.
Several independent observations of the Galactic core suggest hitherto unexplained sources of energy. We show that dark matter in the form of dense antimatter droplets provides a natural site for electron and proton annihilation, providing 511 keV photons, gamma-rays, and energy that sustains thermal X-ray radiation. Such a picture not only identifies the dark matter in our universe, but allows X-ray observations to directly probe the matter distribution in our Galaxy.
We place, by the maximum likelihood method, constraints on a variable dark energy model with the equation of state $w=w_0/[1+b\ln (1+z)]^2$ using some recent observational data, including the new Sne Ia data from the SNLS, the size of baryonic acoustic oscillation peak from SDSS and the CMB data from WMAP3. We find that the SNLS data favor models with $w_0$ around -1, in contrast to the Gold data set which favors a more negative $w_0$. By combining these three databases, we obtain that $\Omega_m=0.27_{-0.038}^{+0.036}$, $w_0=-1.11_{-0.30}^{+0.21}$ and $b=0.31^{+ 0.71}_{-0.31}$ with $\chi^2=110.4$ at the 95% confidence level. Our result suggests that a varying dark energy model and a crossing of the $w = -1$ line are favored, and the present value of the equation of state of dark energy is very likely less than -1.
We present zBootes, a new z-band photometric imaging campaign of 7.62 square degrees in the NOAO Deep Wide-Field Survey (NDWFS) Bootes field. In this paper, all of the images for this survey are released as well as the associated catalogs. The final zBootes catalogs are complete (at the 50% level) to 22.7 mag over 50% of the field. With these depths, the zBootes images should be sensitive to L* galaxies to z~1 over much of the survey area. These data have several possible applications including searching for and characterizing high-redshift quasars and brown dwarfs and providing added constraints to photometric redshift determinations of galaxies and active galaxies to moderate redshift. The zBootes imaging adds photometric data at a new wavelength to the existing wealth of multi-wavelength observations of the NDWFS Bootes field.
Abundance ratios relative to iron for carbon, nitrogen, strontium and barium are presented for a metal-rich main sequence star ([Fe/H]=--0.74) in the globular cluster omega Centauri. This star, designated 2015448, shows depleted carbon and solar nitrogen, but more interestingly, shows an enhanced abundance ratio of strontium [Sr/Fe] ~ 1.6 dex, while the barium abundance ratio is [Ba/Fe]<0.6 dex. At this metallicity one usually sees strontium and barium abundance ratios that are roughly equal. Possible formation scenarios of this peculiar object are considered.
We study the synchrotron flaring behaviour of the blazar 3C 279 and microquasar Cyg X-3. The properties of a typical outburst are derived from the observations by decomposing multi-frequency lightcurves into series of self-similar events. We also discuss the similarities and differences in flaring behaviour of the galactic and extragalactic jets.
For the last three years we have performed a survey for young (<3 Gyrs) giant planets around nearby white dwarfs with HST, Spitzer, and VLT. Direct HST/NICMOS imaging of the seven white dwarfs in the Hyades gave no evidence for companions down to about 10 Jupiter masses and separations larger than 0.5 arcsec (about 25 AU), while VLT/NACO observations revealed a putative companion to a field white dwarf. Second epoch observations with SINFONI on the VLT, however, showed that it is most probably a background star. With IRAC on Spitzer we also found no indications of cool, very low mass companions in our sample of field white dwarfs. The implications of these non-detections are briefly discussed.
We report on the results of a new, sealed, Gas Pixel Detector. The very compact design and the absence of the gas flow system, make this detector substantially ready for use as focal plane detector for future X-ray space telescopes. The instrument brings high sensitivity to X-ray polarimetry, which is the last unexplored field of X-ray astronomy. It derives the polarization information from the track of the photoelectrons that are imaged by a high gain (>1000), fine pitch GEM that matches the pitch of a pixel ASIC which is the collecting anode of the GPD (105k, 50 micron wide, hexagonal cells). The device is able to simultaneously perform good imaging (50-60 micron), moderate spectroscopy (~15% at 6 keV) as well as fast, high rate timing in the 1-10keV range. Moreover, being truly 2D, it is non dispersive and does not require any rotation. The great improvement of sensitivity, at least two orders of magnitude with respect to traditional polarimeters (based on Bragg crystals or Thomson scattering), will allow the direct exploration of the most dramatic objects of the X-ray sky. At the focus of the large mirror area of the XEUS telescope it will be decisive in reaching many of the scientific goals of the mission. With integration times of the order of one day, polarimetry of Active Galactic Nuclei at the per cent level will be possible, making for a real breakthrough in high energy astrophysics.
The search of consistency between nebular and massive star abundances has been a longstanding problem. I briefly review what has been done regarding to this topic, also presenting a recent study focused on the Orion nebula: the O and Si stellar abundances resulting from a detailed and fully consistent spectroscopic analysis of the group of B stars associated with the Orion nebula are compared with the most recent nebular gas-phase results.
The influence of cool galaxy cluster cores on the X-ray luminosity--gravitational mass relation is studied with Chandra observations of 64 clusters in the HIFLUGCS sample. As preliminary results we find (i) a significant offset of cool core (CC) clusters to the high luminosity (or low mass) side compared to non-cool core (NCC) clusters, (ii) a smaller scatter of CC clusters compared to NCC clusters, (iii) a decreasing fraction of CC clusters with increasing cluster mass, (iv) a reduced scatter in the luminosity--mass relation for the entire sample if the luminosity is scaled properly with the central entropy. The implications of these results on the intrinsic scatter are discussed.
Recent CCD observations were performed in the period 1998-2004 for a large sample of visual double and multiple stars selected from the Hipparcos Catalogue and/or from the Gliese Catalogue of Nearby Stars. Accurate astrometric and photometric data allowing to characterize the individual components are provided. These data are confronted to Hipparcos data or to data from an older epoch in order to assess the nature of the observed systems. We simultaneously apply a Moffat-Lorentz profile with a similar shape to all detected components and adjust the profile parameters from which we obtain the relative astrometric position (epoch, position angle, angular separation) as well as differential multi-colour photometry (filters (B)VRI). We thus acquired recent data for 71 visual systems of which 6 are orbital binaries, 27 are nearby and 30 are multiple systems. In three cases, the systems remained unresolved. 23 new components were detected and measured. Two new visual double stars of intermediate separation were also found. The estimated accuracies in relative position are 0.04 deg and 0.01" respectively, while those in differential photometry are of the order of 0.01-0.02 mag in general. The nature of the association of 55 systems is evaluated. New basic binary properties are derived for 20 bound systems. Component colours and masses are provided for two orbital binaries.
4u 0115+63 is one of the most active and best studied Be/X-ray transients. Previous studies of 4u0115+63 have led to the suggestion that it undergoes relatively fast quasi-cyclic activity. However, due to the lack of good coverage of the observations, the variability time scales are uncertain. Our objective is to investigate the long-term behaviour of 4u 0115+63 to confirm its quasi-cyclic nature and to explain its correlated optical/IR and X-ray variability. We have performed optical/IR photometric observations and optical spectroscopic observations of 4u 0115+63 over the last decade with unprecedented coverage. We have focused on the Halpha line variability and the long-term changes of the photometric magnitudes and colours and investigated these changes in correlation with the X-ray activity of the source. results The optical and infrared emission is characterised by cyclic changes with a period of ~ 5 years. This long-term variability is attributed to the state of the circumstellar disc around the Be star companion. Each cycle involves a low state when the disc is very weak or absent and the associated low amplitude variability is orbitally modulated and a high state when a perturbed disc precesses, giving rise to fast and large amplitude photometric changes. X-ray outbursts in 4u 0115+63 come in pairs, i.e., two in every cycle. However, sometimes the second outburst is missing. Our results can be explained within the framework of the decretion disc model. The neutron star acts as the perturbing body, truncating and distorting the disc. The first outburst would occur before the disc is strongly perturbed. The second outburst leads to the dispersal of the disc and marks the end of the perturbed phase.
The existence of older stars within a young star cluster can be interpreted to imply that star formation occurs on time scales longer than a free-fall time of a pre-cluster cloud core. Here the idea is explored that these older stars are not related to the star formation process forming the young star cluster but rather that the orbits of older field stars are focused by the collapsing pre-cluster cloud core. Two effects appear: The focussing of stellar orbits leads to an enhancement of the density of field stars in the vicinity of the centre of the young star cluster. And due to the time-dependent potential of the forming cluster some of these stars can get bound gravitationally to the cluster. These stars exhibit similar kinematical properties as the newly formed stars and can not be distinguished from them on the basis of radial-velocity or proper-motion surveys. Such contaminations may lead to a wrong apparent star-formation history of a young cluster. In the case of the ONC the theoretical number of gravitationally bound older low-mass field stars agrees with the number of observed older low-mass stars.
In light of the three-year data release from WMAP we re-examine the evidence for the ``Axis of Evil''. We discover that previous frequentist methods are not robust with respect to the data-sets available and different treatments of the galactic plane. We identify the cause of the instability and show that this result is not a weakness of the data. This is further confirmed by exhibiting an alternative approach, Bayesian in flavour, and based on a likelihood method and the information criteria. We find strong (and sometimes decisive) evidence for the ``Axis of Evil'' in almost all renditions of the WMAP data. However some significant differences between data-sets remain, and the quantitative aspects of the result depend on the particular information criteria used.
We carried out time-resolved photometric observations of an SU UMa type dwarf
nova, BC UMa, during its superoutburst in February 2003. We detect early
superhumps (or outburst orbital humps) during the first two days of the
outburst. The period of early superhumps is 0.06258(13) d and the amplitude is
0.04 mag. After the early superhump phase, common superhumps with an amplitude
of 0.3 mag and a period of 0.064466(16) d developed. The change rate of
superhump periods was positive through the superoutburst. The superhump period
excess is 3% and we derive a mass ratio of 0.13. This is twice as large as that
of WZ Sge, suggesting that the mechanism of early superhumps in BC UMa is not
the 2:1 resonance which was proposed in WZ Sge.
We have modelled early superhump light curves including irradiation effects
of the accretion disk and secondary star by the white dwarf and accretion disk.
The observed early superhumps can be reproduced when two-armed spirals appear
on the accretion disk.
We have found the long term data taken from AAVSO, VSOLJ and VSNET shows BC
UMa has normal outbursts with a maximum magnitude of V~13 and two types of
superoutbursts: one has a short duration (around 10 days) and faint maximum
magnitude (V~12.5), the other has a long duration (around 20 days) and bright
maximum (V~11-11.5). BC UMa is the first example of dwarf novae showing the two
types of superoutbursts. The supercycle of BC UMa is between 600 days and 1000
days. This is shorter than WZ Sge type dwarf novae and longer than normal SU
UMa type stars. This phenomenon suggests that BC UMa is an intermediate dwarf
nova between WZ Sge and SU UMa.
AIMS: To study the properties of X-ray emissions from young stellar objects (YSOs), through their evolution from Class I to Class III and determine whether Class 0 protostars emit in X-rays. METHODS: A deep Chandra X-ray observation of the Serpens star-forming region was obtained. The Serpens Cloud Core is ideally suited for this type of investigation, being populated by a dense and extremely young cluster whose members are found in all different evolutionary stages, including six well studied Class 0 sources. RESULTS: None of the six Class 0 protostars is detected in our observations, excluding the presence of sources with X-ray luminosities > 0.4 10^30 erg/s (for column densities of the order of 4 10^{23} cm^-2, or A_V ~ 200). A total of 85 X-ray sources are detected and the light curves and spectra of 35 YSOs are derived. There is a clear trend of decreasing absorbing column densities as one moves from Class I to Class III sources, and, possibly, evidence of decreasing plasma temperatures, too. We observe a strong, long-duration, flare from a Class II low-mass star, for which we derive a flaring loop length of the order of 20 stellar radii. We interpret the flaring event as originating from a magnetic flux tube connecting the star to its circumstellar disk. The presence of such a disk is supported by the detection, in the spectrum of this star, of 6.4 keV Fe fluorescent emission.
Various aspects of the high-energy emission from relativistic jets associated wi th compact astrophysical systems are reviewed. The main leptonic and hadronic processes responsible for the production of high-energy $\gamma$-rays, very-high energy neutrinos and ultra-high energy cosmic rays are discussed. Relations between the $\gamma\gamma$ pair production and photomeson production opacities are derived, and their consequences for the relative emission of $\gamma$-rays and neutrinos are examined. The scaling of the size and location of the various emission zones and other quantities with black hole mass and dimensionless luminosity is elucidated. The results are applied to individual classes of objects, including blazars, microquasars and gamma-ray bursts. It is concluded that if baryons are present in the jet at sufficient quantities, then under optimal conditions most systems exhibiting relativistic jets may be detectable by upcoming neutrino telescopes. An exception is the class of TeV blazars, for which $\gamma$-ray observations imply neutrino yields well below detection limit.
We present our latest results for simulation for merger of black hole (BH)-neutron star (NS) binaries in full general relativity which is performed preparing a quasicircular state as initial condition. The BH is modeled by a moving puncture with no spin and the NS by the $\Gamma$-law equation of state with $\Gamma=2$ and corotating velocity field as a first step. The mass of the BH is chosen to be $\approx 3.2 M_{\odot}$ or $4.0M_{\odot}$, and the rest-mass of the NS $\approx 1.4 M_{\odot}$ with relatively large radius of the NS $\approx 13$--14 km. The NS is tidally disrupted near the innermost stable orbit but $\sim 80$--90% of the material is swallowed into the BH and resulting disk mass is not very large as $\sim 0.3M_{\odot}$ even for small BH mass $\sim 3.2M_{\odot}$. The result indicates that the system of a BH and a massive disk of $\sim M_{\odot}$ is not formed from nonspinning BH-NS binaries irrespective of BH mass, although a disk of mass $\sim 0.1M_{\odot}$ is a possible outcome for this relatively small BH mass range as $\sim 3$--4$M_{\odot}$. Our results indicate that the merger of low-mass BH and NS may form a central engine of short-gamma-ray bursts.
We present an analysis of the Evershed effect observed with a resolution of 0.2 arcsec. Using the new Swedish 1-m Solar Telescope and its Littrow spectrograph, we scan a significant part of a sunspot penumbra. Spectra of the non-magnetic line Fe I 7090.4 A allows us to measure Doppler shifts without magnetic contamination. The observed line profiles are asymmetric. The Doppler shift depends on the part of the line used for measuring, indicating that the velocity structure of penumbrae remains unresolved even with our angular resolution. The observed line profiles are properly reproduced if two components with velocities between zero and several km/s co-exist in the resolution elements. Using Doppler shifts at fixed line depths, we find a local correlation between upflows and bright structures, and downflows and dark structures. This association is not specific of the outer penumbra but it also occurs in the inner penumbra. The existence of such correlation was originally reported by Beckers & Schroter (1969), and it is suggestive of energy transport by convection in penumbrae.
Due to dynamical friction stellar mass black holes and neutron stars are expected to form high density cusps in the inner parsec of our Galaxy. These compact remnants, expected to number around 20000, may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. Here we build a simple but detailed time-dependent model of such emission. We find that at least several X-ray sources of this nature should be detectable with Chandra at any one time. Turning this issue around, we also ask a question of what current observational constraints might be telling us about the total number of compact remnants. A cusp with ~ 40 thousand black holes over-predicts the number of discrete sources and the total X-ray luminosity of the inner parsec, and is hence ruled out. Future observations of the distribution and orbits of the cold ionised gas in the inner parsec of Sgr A* will put tighter constraints on the cusp of compact remnants.
The fundamental properties of detached eclipsing binary stars can be measured very accurately, which could make them important objects for constraining the treatment of convection in theoretical stellar models. However, only four or five pieces of information can be found for the average system, which is not enough. We discuss studies of more interesting and useful objects: eclipsing binaries in clusters and eclipsing binaries with pulsating components.
The factors affecting vortex growth in convectively stable protoplanetary
disks are explored using numerical simulations of a two-dimensional
anelastic-gas model which includes baroclinic vorticity production and
radiative cooling. The baroclinic feedback, where anomalous temperature
gradients produce vorticity through the baroclinic term and vortices then
reinforce these temperature gradients, is found to be an important process in
the rate of growth of vortices in the disk. Factors which strengthen the
baroclinic feedback include fast radiative cooling, high thermal diffusion, and
large radial temperature gradients in the background temperature. When the
baroclinic feedback is sufficiently strong, anticyclonic vortices form from
initial random perturbations and maintain their strength for the duration of
the simulation, for over 600 orbital periods.
Based on both simulations and a simple vortex model, we find that the local
angular momentum transport due to a single vortex may be inward or outward,
depending its orientation. The global angular momentum transport is highly
variable in time, and is sometimes negative and sometimes positive. This result
is for an anelastic gas model, and does not include shocks that could affect
angular momentum transport in a compressible-gas disk.
The Taiwan-America Occultation Survey (TAOS) aims to determine the number of small icy bodies in the outer reach of the Solar System by means of stellar occultation. An array of 4 robotic small (D=0.5 m), wide-field (f/1.9) telescopes have been installed at Lulin Observatory in Taiwan to simultaneously monitor some thousand of stars for such rare occultation events. Because a typical occultation event by a TNO a few km across will last for only a fraction of a second, fast photometry is necessary. A special CCD readout scheme has been devised to allow for stellar photometry taken a few times per second. Effective analysis pipelines have been developed to process stellar light curves and to correlate any possible flux changes among all telescopes. A few billion photometric measurements have been collected since the routine survey began in early 2005. Our preliminary result of a very low detection rate suggests a deficit of small TNOs down to a few km size, consistent with the extrapolation of some recent studies of larger (30--100 km) TNOs.
The formation of vortices in protoplanetary disks is explored via pseudo-spectral numerical simulations of an anelastic-gas model. This model is a coupled set of equations for vorticity and temperature in two dimensions which includes baroclinic vorticity production and radiative cooling. Vortex formation is unambiguously shown to be caused by baroclinicity because (1) these simulations have zero initial perturbation vorticity and a nonzero initial temperature distribution; and (2) turning off the baroclinic term halts vortex formation, as shown by an immediate drop in kinetic energy and vorticity. Vortex strength increases with: larger background temperature gradients; warmer background temperatures; larger initial temperature perturbations; higher Reynolds number; and higher resolution. In the simulations presented here vortices form when the background temperatures are $\sim 200K$ and vary radially as $r^{-0.25}$, the initial vorticity perturbations are zero, the initial temperature perturbations are 5% of the background, and the Reynolds number is $10^9$. A sensitivity study consisting of 74 simulations showed that as resolution and Reynolds number increase, vortices can form with smaller initial temperature perturbations, lower background temperatures, and smaller background temperature gradients. For the parameter ranges of these simulations, the disk is shown to be convectively stable by the Solberg-H{\o}iland criteria.
We present causal and positional evidence of triggered star formation in bright-rimmed clouds in OB associations, e.g., Ori OB1, and Lac OB1, by photoionization. The triggering process is seen also on a much larger scale in the Orion-Monoceros Complex by the Orion-Eridanus Superbubble. We also show how the positioning of young stellar groups surrounding the H II region associated with Trumpler 16 in Carina Nebula supports the triggering process of star formation by the collect-and-collapse scenario.
Active Galactic Nuclei (AGN) can be probed by at different regions of the electromagnetic spectrum: e.g., radio observations reveal the nature of their relativistic jets and their magnetic fields, and complementarily, X-ray observations give insight into the changes in the accretion disk flows. Here we present an overview over the AGN research and results from an ongoing multi-band campaign on the active galaxy NGC1052. Beyond these studies, we address the latest technical developments and its impact in the AGN field: the Square Kilometre Array (SKA), a new radio interferometer planned for the next decade, and the oncoming X-ray and gamma-ray missions.
Using a series of three-dimensional, hydrodynamic simulations on an adaptive grid, we have performed a systematic study on the effect of bubble-induced motions on metallicity profiles in clusters of galaxies. In particular, we have studied the dependence on the bubble size and position, the recurrence times of the bubbles, the way these bubbles are inflated and the underlying cluster profile. We find that in hydrostatic cluster models, the resulting metal distribution is very elongated along the direction of the bubbles. Anisotropies in the cluster or ambient motions are needed if the metal distribution is to be spherical. In order to parametrise the metal transport by bubbles, we compute effective diffusion coefficients. The diffusion coefficients inferred from our simple experiments lie at values of around $\sim 10^{29}$ cm$^2$s$^{-1}$ at a radius of 10 kpc. The runs modelled on the Perseus cluster yield diffusion coefficients that agree very well with those inferred from observations.
We present some results of the new MORGANA model for the rise of galaxies and active nuclei, and show that the improved physical motivation of the description of star formation and feedback allows to get hints on the physical processes at play. We propose that the high level of turbulence in star-forming bulges is at the base of the observed downsizing of AGNs. In this framework it is also possible to reproduce the recently obtained evidence that most low-redshift accretion is powered by relatively massive, slowly accreting black holes. Besides, we notice that many galaxy formation models (including MORGANA) fail to reproduce a basic observable, namely the number density of 10^{11} Msun galaxies at z~1, as traced by the GOODS-MUSIC sample. This points to a possibly missing ingredient in the modeling of stellar feedback.
The effect of gravitational microlensing on the intensity of gravitational radiation as it propagates through an inhomogeneous medium containing small-scale structure is considered. Lensing by both stars and a power law spectrum of density perturbations is examined. The long wavelengths characteristic of gravitational radiation mandate a statistical, physical-optics approach to treat the effect of stellar microlensing. A model for the mass power spectrum of a starfield, including the effects of clustering and allowing for a distribution of stellar masses, is constructed and used to determine both the amplitude of fluctuations in the gravitational wave strain and its associated temporal fluctuation spectrum. For a uniformly distributed starfield the intensity variance scales linearly with stellar density, sigma, but is enhanced by a factor >sigma r_F^2 when clustering is important, where r_F is the Fresnel scale. The effect of lensing by a power law mass spectrum is also considered. For spectra with indices steeper than -2 the wave amplitude exhibits rms fluctuations 1.3 < Delta Sigma^2 >^0.25 (D_eff/1 Gpc)^0.5 %, where <Delta Sigma^2> is the variance in the mass surface density measured in M_sun^2 pc^-4 and D_eff is the effective distance to the lensing medium. For shallower spectra the amplitude of the fluctuations depends additionally on the inner length scale and power law index of the density fluctuations. The intensity fluctuations are dominated by temporal fluctuations on too long a timescale to be observable.
We consider the flow of an electrically conducting fluid between differentially rotating cylinders, in the presence of an externally imposed toroidal field B_0 (r_i/r) e_phi. It is known that the classical, axisymmetric magnetorotational instability does not exist for such a purely toroidal imposed field. We show here that a non-axisymmetric magnetorotational instability does exist, having properties very similar to the axisymmetric magnetorotational instability in the presence of an axial field.
We have mapped the central 10'x10' of the dense core TMC-1C at 450, 850 and 1200 microns using SCUBA on the James Clerk Maxwell Telescope and MAMBO on the IRAM 30m telescope. We show that although one can, in principle, use images at these wavelengths to map the emissivity spectral index, temperature and column density independently, noise and calibration errors would have to be less than 2% to accurately derive these three quantities from a set of three emission maps. Because our data are not this free of errors, we use our emission maps to fit the dust temperature and column density assuming a constant value of the emissivity spectral index and explore the effects of noise on the derived physical parameters. We find that the derived extinction values for TMC-1C are large for a starless core (80 mag Av), and the derived temperatures are low (6 K) in the densest regions of the core, using our derived value of beta = 1.8.
We have found empirically that the radio loudness of AGN can be understood as function of both the X-ray and optical luminosity. This way of considering the radio loudness was inspired by the hardness-intensity diagrams for X-ray binaries, in which objects follow a definite track with changes to their radio properties occurring in certain regions. We generalize the hardness-intensity diagram to a "disk-fraction luminosity diagram", which can be used to classify the accretion states both of X-ray binaries and of AGN. Using a sample of nearly 5000 SDSS quasars with ROSAT matches, we show that an AGN is more likely to have a high radio:optical flux ratio when it has a high total luminosity or a large contribution from X-rays. Thus, it is necessary to take into account both the optical and the X-ray properties of quasars in order to understand their radio loudness. The success of categorizing quasars in the same way as X-ray binaries is further evidence for the unification of accretion onto stellar-mass and supermassive compact objects.
We review the dynamics of magnetic spacetimes and then consider the effects of magnetic fields on density perturbations in the universe. Using covariant techniques, we refine and extend earlier work. We provide the magnetohydrodynamic equations that describe inhomogeneous magnetic cosmologies in full general relativity. Specialising this system to perturbed Friedmann-Robertson-Walker models, we examine the effects of the field on the expansion dynamics and on the growth of density perturbations, including non-adiabatic modes. We obtain analytic solutions for the linear evolution of these distortions during the inflationary, radiation, and dust eras and calculate the scale of the `magnetic' Jeans length.
A popular aspect of black holes physics is the mathematical analogy between their laws, coming from general relativity and the laws of thermodynamics. The analogy is achieved by identifying a suitable set of observables, precisely: \emph{(a)} $E=M$ (being $E$ the thermodynamic free energy and $M$ the mass of the BH), \emph{(b)} $T=\alpha \kappa $ (with $T$ the absolute temperature, $\kappa $ the so-called surface gravity on event horizon and $% \alpha $ a suitable dimensional constant) and \emph{(c)} $S=(1/8\pi \alpha)A $ (where $S$ is the thermodynamic entropy of the black hole and $A$ the surface of the event horizon). However, despite numerous investigations and efforts spent on the subject, the theoretical foundations of such identifications between physical quantities belonging to apparently unrelated frameworks are not yet clear. The goal of this work is to provide the contribution to the black hole entropy, coming from matter in the black hole exterior. We propose a classical solution for the kinetic description of matter falling into a black hole, which permits to evaluate both the kinetic entropy and the entropy production rate of classical infalling matter at the event horizon. The formulation is based on a relativistic kinetic description for classical particles in the presence of an event horizon. An H-theorem is established which holds for arbitrary models of black holes and is valid also in the presence of contracting event horizons.
Integrated Sachs-Wolfe (ISW) measurements, which involve cross-correlating the CMB with the foreground large-scale structure (e.g. galaxies/quasars), have proven to be an interesting probe of dark energy. We show that magnification bias, which is the inevitable modulation of the foreground number counts by gravitational lensing, alters both the shape and amplitude of the observed ISW signal. This is true especially at high redshifts because (1) the intrinsic galaxy-temperature signal diminishes greatly back in the matter dominated era, (2) the lensing efficiency increases with redshift and (3) the number count slope generally steepens with redshift in a magnitude limited sample. At z >~ 2, the magnification-temperature correlation dominates over the intrinsic galaxy-temperature correlation and causes the observed ISW signal to increase with z, despite dark energy subdominance -- a result of the fact that magnification probes structures between the observer and the sources. Ignoring magnification bias can then lead to erroneous conclusions about dark energy. While the lensing modulation opens up an interesting high z window for ISW measurements, high z measurements are not expected to add much new information to low z ones if dark energy is the cosmological constant. This is because lensing introduces significant covariance across redshifts. The most compelling reason to pursue high z ISW measurements is to look for a potential surprise such as early dark energy domination or the signature of modified gravity. We conclude with a discussion of existing measurements, the highest z of which is at the margin of being sensitive to magnification bias. We also develop a formalism which might be of general interest: to predict biases in estimating parameters when certain physical effects are ignored in interpreting data.