lanl.arXiv.org > astro-ph

Astrophysics

astro-ph new abstracts, Fri, 2 Jun 06 00:00:14 GMT
0606001 -- 0606034 received

astro-ph/0606001 [abs, ps, pdf, other] :

Title: Estimators for local non-Gaussianities
Authors: Paolo Creminelli (ICTP, Trieste), Leonardo Senatore (MIT), Matias Zaldarriaga (Harvard-Smithsonian Ctr. Astrophys. & Harvard U., Phys. Dept.)
Comments: 25 pages

We study the Likelihood function of data given f_NL for the so-called local type of non-Gaussianity. In this case the curvature perturbation is a non-linear function, local in real space, of a Gaussian random field. We compute the Cramer-Rao bound for f_NL and show that for small values of f_NL the 3-point function estimator saturates the bound and is equivalent to calculating the full Likelihood of the data. However, for sufficiently large f_NL, the naive 3-point function estimator has a much larger variance than previously thought. In the limit in which the departure from Gaussianity is detected with high confidence, error bars on f_NL only decrease as 1/ln Npix rather than Npix^-1/2 as the size of the data set increases. We identify the physical origin of this behavior and explain why it only affects the local type of non-Gaussianity, where the contribution of the first multipoles is always relevant. We find a simple improvement to the 3-point function estimator that makes the square root of its variance decrease as Npix^-1/2 even for large f_NL, asymptotically approaching the Cramer-Rao bound. We show that using the modified estimator is practically equivalent to computing the full Likelihood of f_NL given the data. Thus other statistics of the data, such as the 4-point function and Minkowski functionals, contain no additional information on f_NL. In particular, we explicitly show that the recent claims about the relevance of the 4-point function are not correct. By direct inspection of the Likelihood, we show that the data do not contain enough information for any statistic to be able to constrain higher order terms in the relation between the Gaussian field and the curvature perturbation, unless these are orders of magnitude larger than the size suggested by the current limits on f_NL.

 

astro-ph/0606002 [abs, ps, pdf, other] :

Title: The X-ray Properties of Optically-Selected Galaxy Clusters
Authors: Xinyu Dai, Christopher S. Kochanek, Nicholas D. Morgan (The Ohio State University)
Comments: 33 pages, 14 figures, submitted to ApJ

We stacked the X-ray data from the ROSAT All Sky Survey for over 4,000 clusters selected from the 2MASS catalog and divided into five richness classes. We detected excess X-ray emission over background at the center of the stacked images in all five richness bins. The interrelationships between the mass, X-ray temperature and X-ray luminosity of the stacked clusters agree well with those derived from catalogs of X-ray clusters. Poisson variance in the number of galaxies occupying halos of a given mass leads to significant differences between the average richness at fixed mass and the average mass at fixed richness that we can model relatively easily using a simple model of the halo occupation distribution. These statistical effects probably explain recent results in which optically-selected clusters lie on the same X-ray luminosity-temperature relations as local clusters but have lower optical richnesses than observed for local clusters with the same X-ray properties. When we further binned the clusters by redshift, we did not find significant redshift-dependent biases in the sense that the X-ray luminosities for massive clusters of fixed optical richness show little dependence on redshift beyond that expected from the effects of Poisson fluctuations. Our results demonstrate that stacking of RASS data from optically selected clusters can be a powerful test for biases in cluster selection algorithms.

 

astro-ph/0606003 [abs, ps, pdf, other] :

Title: Tachyon dark energy models: dynamics and constraints
Authors: Gianluca Calcagni, Andrew R. Liddle
Comments: 9 pages, 4 figures

We explore the dynamics of dark energy models based on a Dirac-Born-Infeld (DBI) tachyonic action, studying a range of potentials. We numerically investigate the existence of tracking behaviour and determine the present-day value of the equation of state parameter and its running, which are compared with observational bounds. We find that tachyon models have quite similar phenomenology to canonical quintessence models. While some potentials can be selected amongst many possibilities and fine-tuned to give viable scenarios, there is no apparent advantage in choosing a DBI scalar field instead of a Klein-Gordon one.

 

astro-ph/0606004 [abs, ps, pdf, other] :

Title: The bright optical flash from GRB 060117
Authors: Martin Jelínek, Michael Prouza, Petr Kubánek, René Hudec, Martin Nekola, Jan Rídký, Jirí Grygar, Martina Bohácová, Alberto J. Castro-Tirado, Javier Gorosabel, Miroslav Hrabovský, Dusan Mandát, Dalibor Nosek, Libor Nozka, Miroslav Palatka, Shashi B. Pandey, Miroslav Pech, Petr Schovánek, Radomír Smída, Petr Trávnícek, Antonio de Ugarte Postigo, Stanislav Vítek
Comments: Accepted to A&A, 4 pages

We present a discovery and observation of an extraordinarily bright prompt optical emission of the GRB 060117 obtained by a wide-field camera atop the robotic telescope FRAM of the Pierre Auger Observatory from 2 to 10 minutes after the GRB. We found rapid average temporal flux decay of alpha = -1.7 +- 0.1 and a peak brightness R = 10.1 mag. Later observations by other instruments set a strong limit on the optical and radio transient fluxes, unveiling an unexpectedly rapid further decay. We present an interpretation featuring a relatively steep electron-distribution parameter p ~ 3.0 and providing a straightforward solution for the overall fast decay of this optical transient as a transition between reverse and forward shock.

 

astro-ph/0606005 [abs, ps, pdf, other] :

Title: On the efficiency of Fermi acceleration at relativistic shocks
Authors: Martin Lemoine (IAP), Guy Pelletier (LAOG), Benoit Revenu (SUBATECH)
Comments: 4 pages, 2 figures; to appear in ApJ Letters

It is shown that Fermi acceleration at an ultra-relativistic shock wave cannot operate on a particle for more than 1 1/2 Fermi cycle (i.e., u -> d -> u -> d) if the particle Larmor radius is much smaller than the coherence length of the magnetic field on both sides of the shock, as is usually assumed. This conclusion is shown to be in excellent agreement with recent numerical simulations. We thus argue that efficient Fermi acceleration at ultra-relativistic shock waves requires significant non-linear processing of the far upstream magnetic field with strong amplification of the small scale magnetic power. The streaming or transverse Weibel instabilities are likely to play a key role in this respect.

 

astro-ph/0606006 [abs, ps, pdf, other] :

Title: Arc sensitivity to cluster ellipticity, asymmetries and substructures
Authors: Massimo Meneghetti, Rodolfo Argazzi, Francesco Pace, Lauro Moscardini, Klaus Dolag, Matthias Bartelmann, Guoliang Li, Masamune Oguri
Comments: 15 pages, 16 figures. Version with full resolution images can be found at this http URL

We investigate how ellipticity, asymmetries and substructures separately affect the ability of galaxy clusters to produce strong lensing events, i.e. gravitational arcs, and how they influence the arc morphologies and fluxes. This is important for those studies aiming, for example, at constraining cosmological parameters from statistical lensing, or at determining the inner structure of galaxy clusters through gravitational arcs. We do so by creating two-dimensional gradually smoothed, differently elliptical and asymmetric versions of some numerical models. On average, we find that the contributions of ellipticity, asymmetries and substructures amount to ~40%, ~10% and ~30% of the total strong lensing cross section, respectively. However, our analysis shows that substructures play a more important role in less elliptical and asymmetric clusters, even if located at large distances from the cluster centers (~1Mpc/h). Conversely, their effect is less important in highly asymmetric lenses. The morphology, position and flux of individual arcs are strongly affected by the presence of substructures in the clusters. Removing substructures on spatial scales <~50kpc/h, roughly corresponding to mass scales <~5 10^{10}M_\odot/h, alters the image multiplicity of ~35% of the sources used in the simulations and causes position shifts larger than 5'' for ~40% of the arcs longer than 5''. We conclude that any model for cluster lens cannot neglect the effects of ellipticity, asymmetries and substructures. On the other hand, the high sensitivity of gravitational arcs to deviations from regular, smooth and symmetric mass distributions suggests that strong gravitational lensing is potentially a powerfull tool to measure the level of substructures and asymmetries in clusters.

 

astro-ph/0606007 [abs, ps, pdf, other] :

Title: The Large-Scale Structure view on the Galaxy-Quasar-AGN connection
Authors: Manuela Magliocchetti
Comments: 6 pages, 8 figures, proceedings of the XLIst Rencontres de Moriond 'From Dark Halos to Light', L.Tresse, S.Maurogordato and J. Tran Than h Van editors

Combined investigations of the clustering properties of galaxies of different spectral type and high-redshift quasars strongly suggest local ellipticals to be the parent population of optically bright Active Galactic Nuclei (AGN). However, the picture gets more blurred when one extends the analysis to that class of AGNs which show enhanced radio emission. Objects belonging to this class in fact are found to be associated with structures which are about an order of magnitude more massive than those that host radio-quiet AGNs. Also, masses for the black holes engines of radio-enhanced AGN emission turn out to be systematically higher than those which fuel 'normal' quasars. On the other hand, the level of radio-activity in radio-luminous objects does not seem to be connected with black hole/host galaxy mass, at variance with what found in the optical case. These results, together with evidences for different cosmological evolutions of different types of AGNs pose a serious challenge to all those models aiming at providing a unified picture for black hole-powered sources.

 

astro-ph/0606008 [abs, pdf] :

Title: Probing the Low-Luminosity XLF in Normal Elliptical Galaxies
Authors: D.-W. Kim, G. Fabbiano, V. Kalogera, A. R. King, S. Pellegrini, G. Trinchieri, S. E. Zepf, A. Zezas, L. Angelini, R. L. Davies, J. S. Gallagher
Comments: 18 pages, 6 figures, submitted to ApJ

We present the first low luminosity (LX > 5 - 10 1036 erg s-1) X-ray luminosity functions (XLFs) of low-mass X-ray binaries (LMXBs) determined for two typical old elliptical galaxies, NGC 3379 and NGC 4278. Because both galaxies contain little diffuse emission from hot ISM and no recent significant star formation (hence no high-mass X-ray binary contamination), they provide two of the best homogeneous sample of LMXBs. With 110 and 140 ks Chandra ACIS S3 exposures, we detect 59 and 112 LMXBs within the D25 ellipse of NGC 3379 and NGC 4278, respectively. The resulting XLFs are well represented by a single power-law with a slope (in a differential form) of 1.9 0.1. In NGC 4278, we can exclude the break at LX ~ 5 x 1037 erg s-1 that was recently suggested to be a general feature of LMXB XLFs. In NGC 3379 instead we find a localized excess over the power law XLF at ~4 x 1037 erg s-1, but with a marginal significance of ~1.6s. Because of the small number of luminous sources, we cannot constrain the high luminosity break (at 5 x 1038 erg s-1) found in a large sample of early type galaxies. While the optical luminosities of the two galaxies are similar, their integrated LMXB X-ray luminosities differ by a factor of 4, consistent with the relation between the X-ray to optical luminosity ratio and the globular cluster specific frequency.

 

astro-ph/0606009 [abs, ps, pdf, other] :

Title: The Eccentricity-Mass Distribution of Exoplanets: Signatures of Different Formation Mechanisms?
Authors: Ignasi Ribas (CSIC/IEEC, Spain), Jordi Miralda-Escude (ICREA/IEEC, Spain, and IAS Princeton, USA)
Comments: 7 pages, 4 figures, submitted to A&A

We examine the distributions of eccentricity and host star metallicity of exoplanets as a function of their mass. Planets with M sin i >~ 4 M_J have an eccentricity distribution consistent with that of binary stars, while planets with M sin i <~ 4 M_J are less eccentric than binary stars and more massive planets. In addition, host star metallicities decrease with planet mass. The statistical significance of both of these trends is only marginal with the present sample of exoplanets. To account for these trends, we hypothesize that there are two populations of gaseous planets: the low-mass population forms by gas accretion onto a rock-ice core in a circumstellar disk and is more abundant at high metalliticities, and the high-mass population forms directly by fragmentation of a pre-stellar cloud. Planets of the first population form in initially circular orbits and grow their eccentricities later, and may have a mass upper limit from the total mass of the disk that can be accreted by the core. The second population may have a mass lower limit resulting from opacity-limited fragmentation. This would roughly divide the two populations in mass, although they would likely overlap over some mass range. If most objects in the second population form before the pre-stellar cloud becomes highly opaque, they would have to be initially located in orbits larger than ~30 AU, and would need to migrate to the much smaller orbits in which they are observed. The higher mean orbital eccentricity of the second population might be caused by the larger required intervals of radial migration, and the brown dwarf desert might be due to the inability of high-mass brown dwarfs to migrate inwards sufficiently in radius.

 

astro-ph/0606010 [abs, ps, pdf, other] :

Title: DEMON: a Proposal for a Satellite-Borne Experiment to study Dark Matter and Dark Energy
Authors: The Alpbach-Graz collaboration
Comments: 12 pages, 3 figures, accepted for publication in the SPIE conference proceedings

We outline a novel satellite mission concept, DEMON, aimed at advancing our comprehension of both dark matter and dark energy, taking full advantage of two complementary methods: weak lensing and the statistics of galaxy clusters. We intend to carry out a 5000 sqdeg combined IR, optical and X-ray survey with galaxies up to a redshift of z~2 in order to determine the shear correlation function. We will also find ~100000 galaxy clusters, making it the largest survey of this type to date. The DEMON spacecraft will comprise one IR/optical and eight X-ray telescopes, coupled to multiple cameras operating at different frequency bands. To a great extent, the technology employed has already been partially tested on ongoing missions, therefore ensuring improved reliability.

 

astro-ph/0606011 [abs, ps, pdf, other] :

Title: Smooth Light Curves from a Bumpy Ride: Relativistic Blast Wave Encounters a Density Jump
Authors: Ehud Nakar, Jonathan Granot
Comments: 16 pages, 14 figures

Some GRB afterglow light curves show significant variability, which often includes episodes of rebrightening. This was attributed in several cases to large fluctuations in the external density. Here we examine the effect of a sharp increase in the external density on the light curve by considering, for the first time, a full treatment of both the hydrodynamic evolution and the radiation in this scenario. To this end we develop a semi-analytic model for the light curve and carry out several elaborate numerical simulations using a 1D hydrodynamic code together with a synchrotron radiation code.Two spherically symmetric cases are explored in detail: a density jump in a uniform external medium, and a wind termination shock. Contrary to previous works, we find that even a sharp (step function) and large (by a factor of $a>>1$) increase in the external density does not produce sharp features in the light curve, and cannot account for significant temporal variability. For a wind termination shock, the light curve smoothly transitions between the asymptotic power-laws over about one decade in time, and there is no rebrightening in the optical or X-rays to serve as a clear observational signature. For a sharp jump in a uniform density we find that the maximal deviation $\Delta\alpha_max$ of the temporal decay index $\alpha$ from its asymptotic value, is bounded (e.g., $\Delta\alpha_max <0.4$ for $a=10$); $\Delta\alpha_max$ slowly increases with $a$, converging to $\Delta\alpha_{max} \approx 1$ at very large $a$ values. Therefore, no optical rebrightening is expected here as well. In the X-rays, while the asymptotic flux is unaffected by the density jump, the fluctuations in $\alpha$ are found to be comparable to those in the optical. Finally, we discuss the implications of our results for the origin of the fluctuations in several afterglows.

 

astro-ph/0606012 [abs, ps, pdf, other] :

Title: Refining a relativistic, hydrodynamic solver: admitting ultra-relativistic flows
Authors: J. P. Bernstein (1), P. A. Hughes (1) ((1) University of Michigan, Ann Arbor)
Comments: 15 pages, 4 figures, submitted to SIAM Journal on Numerical Analysis (SINUM). Note that Figure 2 is a low-resolution version for Astro-ph. A pdf of the paper with the full-resolution version is available at this http URL

We have undertaken the simulation of hydrodynamic flows with bulk Lorentz factors in the range 10^2--10^6. We discuss the application of an existing relativistic, hydrodynamic primitive-variable recovery algorithm to a study of pulsar winds, and, in particular, the refinement necessary to admit such ultra-relativistic flows. We show that the use of an analytical quartic root finder is required for Lorentz factors above 10^2, but that an iterative quartic root finder, which is known to be robust for Lorentz factors up to at least 25, offers a 24% speed advantage. We demonstrate the existence of a simple diagnostic allowing for a hybrid primitives recovery algorithm that includes an automatic, real-time toggle between the iterative and analytical methods. We further determine the accuracy of the iterative and hybrid algorithms for a comprehensive selection of input parameters.

 

astro-ph/0606013 [abs, ps, pdf, other] :

Title: Spiral stellar density waves and the flattening of abundance gradients in the warm gas component of spiral galaxies
Authors: E. I. Vorobyov (The University of Western Ontario)
Comments: 10 pages, 12 figures, accepted for publication in MNRAS

Motivated by recent observations of plateaus and minima in the radial abundance distributions of heavy elements in the Milky Way and some other spiral galaxies, we propose a dynamical mechanism for the formation of such features around corotation. Our numerical simulations show that the non-axisymmetric gravitational field of spiral density waves generates cyclone and anticylone gas flows in the vicinity of corotation. The anticyclones flatten the pre-existing negative abundance gradients by exporting many more atoms of heavy elements outside corotation than importing inside it. This process is very efficient and forms plateaus of several kiloparsec in size around corotation after two revolution periods of a galaxy. The strength of anticyclones and, consequently, the sizes of plateaus depend on the pitch angle of spiral arms and are expected to increase along the Hubble sequence.

 

astro-ph/0606014 [abs, ps, pdf, other] :

Title: Coherent synchrotron emission from cosmic ray air showers
Authors: Qinghuan Luo
Comments: 8 pages, 10 figures, accepted for publication in MNRAS

Coherent synchrotron emission by particles moving along semi-infinite tracks is discussed, with a specific application to radio emission from air showers induced by high-energy cosmic rays. It is shown that in general, radiation from a particle moving along a semi-infinite orbit consists of usual synchrotron emission and modified impulsive bremsstrahlung. The latter component is due to the instantaneous onset of the curved trajectory of the emitting particle at its creation. Inclusion of the bremsstrahlung leads to broadening of the radiation pattern and a slower decay of the spectrum at the cut-off frequency than the conventional synchrotron emission. Possible implications of these features for air shower radio emission are discussed.

 

astro-ph/0606015 [abs, ps, pdf, other] :

Title: Substructure around M31 : Evolution and Effects
Authors: Jean-Rene Gauthier (1), John Dubinski (1), Lawrence M. Widrow (2) ((1) University of Toronto, (2) Queen's University)
Comments: 45 pages, 18 figures. To be submitted to the Astrophysical Journal

We investigate the evolution of a population of 100 dark matter satellites orbiting in the gravitational potential of a realistic model of M31. We find that after 10 Gyr, seven subhalos are completely disrupted by the tidal field of the host galaxy. The remaining satellites suffer heavy mass loss and overall, 75% of the mass initially in the subhalo system is tidally stripped. Not surprisingly, satellites with pericentric radius less than 30 kpc suffer the greatest stripping and leave a complex structure of tails and streams of debris around the host galaxy. Assuming that the most bound particles in each subhalo are kinematic tracers of stars, we find that the halo stellar population resulting from the tidal debris follows an r^{-3.5} density profile at large radii. We construct B-band photometric maps of stars coming from disrupted satellites and find conspicuous features similar both in morphology and brightness to the observed Giant Stream around Andromeda. An assumed star formation efficiency of 5-10% in the simulated satellite galaxies results in good agreement with the number of M31 satellites, the V-band surface brightness distribution, and the brightness of the Giant Stream. During the first 5 Gyr, the bombardment of the satellites heats and thickens the disk by a small amount. At about 5 Gyr, satellite interations induce the formation of a strong bar which, in turn, leads to a significant increase in the velocity dispersion of the disk.

 

astro-ph/0606016 [abs, ps, pdf, other] :

Title: Magellan Spectroscopy of AGN Candidates in the COSMOS Field
Authors: Jonathan R. Trump, Chris D. Impey, Patrick J. McCarthy, Martin Elvis, John P. Huchra, Marcella Brusa, Gunther Hasinger, Eva Schinnerer, Peter Capak, Simon J. Lilly, Nick Z. Scoville
Comments: 19 pages, 14 figures. Accepted to ApJS special COSMOS issue. The full electronic version of Table 2 can be found at this http URL

We present spectroscopic redshifts for the first 466 X-ray and radio-selected AGN targets in the 2 deg^2 COSMOS field. Spectra were obtained with the IMACS instrument on the Magellan (Baade) telescope, using the nod-and-shuffle technique. We identify a variety of Type 1 and Type 2 AGN, as well as red galaxies with no emission lines. Our redshift yield is 72% down to i_AB=24, although the yield is >90% for i_AB<22. We expect the completeness to increase as the survey continues. When our survey is complete and additional redshifts from the zCOSMOS project are included, we anticipate ~1100 AGN with redshifts over the entire COSMOS field. Our redshift survey is consistent with an obscured AGN population that peaks at z~0.7, although further work is necessary to disentangle the selection effects.

 

astro-ph/0606017 [abs, ps, pdf, other] :

Title: High-energy Emission from Pulsar Magnetospheres
Authors: Kouichi Hirotani
Comments: 16 pages, 9 figures, Mod. Phys. Lett. A in press (Brief Review)

A synthesis of the present knowledge on gamma-ray emission from the magnetosphere of a rapidly rotating neutron star is presented, focusing on the electrodynamics of particle accelerators. The combined curvature, synchrotron, and inverse-Compton emission from ultra-relativistic positrons and electrons, which are created by two-photon and/or one-photon pair creation processes, or emitted from the neutron-star surface, provide us with essential information on the properties of the accelerator -- electric potential drop along the magnetic field lines. A new accelerator model, which is a mixture of traditional inner-gap and outer gap models, is also proposed, by solving the Poisson equation for the electrostatic potential together with the Boltzmann equations for particles and gamma-rays in the two-dimensional configuration and two-dimensional momentum spaces.

 

astro-ph/0606018 [abs, pdf] :

Title: VZ Velorum: 116 years of a Mira star
Authors: L.L. Kiss
Comments: 8 pages, 4 figures, to appear in JAAVSO

Using the Harvard College Observatory Photographic Plate Collection and recent CCD observations by the ASAS project we have reconstructed the light variations of the southern pulsating red giant star VZ Velorum between 1890 and early 2006. Contrary to an early report on its low-amplitude semiregular nature, we found a relatively stable Mira-like light curve with a mean period of 318 days and amplitude up to 7 magnitudes. The latest observations show evidence for a slightly shorter period (312 days). However, the difference does not exceed the intrinsic period jitter often seen in Mira type variables.

 

astro-ph/0606019 [abs, ps, pdf, other] :

Title: The HII Region of a Primordial Star
Authors: Tom Abel (KIPAC, Stanford), John H. Wise (KIPAC, Stanford), Greg L. Bryan (Columbia)
Comments: 4 pages, 2 figures. Submitted to ApJL 09 April 2006. Revised manuscript. Comments welcome. Supplementary information and animations at this http URL

The concordance model of cosmology and structure formation predicts the formation of isolated very massive stars at high redshifts in dark matter dominated halos of 10^5 to 10^6 Msun. These stars photo-ionize their host primordial molecular clouds, expelling all the baryons from their halos. When the stars die, a relic HII region is formed within which large amounts of molecular hydrogen form which will allow the gas to cool efficiently when gravity assembles it into larger dark matter halos. The filaments surrounding the first star hosting halo are largely shielded and provide the pathway for gas to stream into the halo when the star has died. We present the first fully three dimensional cosmological radiation hydrodynamical simulations that follow all these effects. A novel adaptive ray casting technique incorporates the time dependent radiative transfer around point sources. This approach is fast enough so that radiation transport, kinetic rate equations, and hydrodynamics are solved self-consistently. It retains the time derivative of the transfer equation and is explicitly photon conserving. This method is integrated with the cosmological adaptive mesh refinement code enzo, and runs on distributed and shared memory parallel architectures. Where applicable the three dimensional calculation not only confirm expectations from earlier one dimensional results but also illustrate the multi-fold hydrodynamic complexities of HII regions. In the absence of stellar winds the circumstellar environments of the first supernovae and putative early gamma-ray bursts will be of low density ~1 cm^-3. Albeit marginally resolved, ionization front instabilities lead to cometary and elephant trunk like small scale structures reminiscent of nearby star forming regions.

 

astro-ph/0606020 [abs, ps, pdf, other] :

Title: Ongoing Star Formation in the BL Lacertae object PKS 2005-489
Authors: A.Bressan, R.Falomo, J.R. Valdes, R.Rampazzo
Comments: Accepted for publication by ApJL

We present VLT long slit optical spectroscopy of the luminous BL Lacertae object PKS 2005-489. The high signal-to-noise ratio and the good spatial resolution of the data allow us to detect the signatures of ongoing star formation in an extended rotating ring, at ~4 kpc from the nucleus. We find that the ring is almost perpendicular to the radio axis and its total star formation rate is ~1 MSol/yr. We briefly discuss the concomitant presence of recent star formation and nuclear activity.

 

astro-ph/0606021 [abs, ps, pdf, other] :

Title: High-resolution X-ray spectroscopy of the Narrow line Seyfert 1 galaxy NGC 4051 with Chandra LETGS
Authors: M. Fenovcik, J.S. Kaastra, E. Costantini, K.C. Steenbrugge, F. Verbunt
Comments: 11 pages, 6 figures, submitted to Astronomy and Astrophysics

We study the absorption and emission properties of the photoionised gas near one of the brightest and most variable AGN, the Seyfert galaxy NGC 4051, in order to constrain the geometry, dynamics and ionisation structure of the outflow. We analyse two observations taken with the Low Energy Transmission Grating Spectrometer (LETGS) of Chandra. We study the spectra of both observations and investigate the spectral response to a sudden, long-lasting flux decrease of a factor of 5 that occurred during the second observation. We confirm the preliminary detection of a highly ionised component with an outflow velocity of -4500 km/s, one of the highest velocity outflow components seen in a Seyfert 1 galaxy. The sudden drop in intensity by a factor of five during the second observation causes a drop in ionisation parameter of a similar magnitude in the strongest and main ionisation component (v = -610 km/s), allowing us for the first time to determine the recombination time of this component and thereby its distance in a robust way. We find an upper limit to the distance of 10^15 m, ruling out an origin in the narrow emission line region. In addition, an emission component producing strong radiative recombination continua of C VI and C V appears during the low state. This can be explained by emission from an ionised skin of the accretion disk at a distance of only ~4x10^12 m from the black hole. Finally, the spectra contain a broad relativistic O VIII line with properties similar to what was found before in this source with XMM-Newton; this line has disappeared during the low flux state, consistent with the disappearance of the inner part of the accretion disk during that low flux state. (abridged)

 

astro-ph/0606022 [abs, ps, pdf, other] :

Title: Necessary and sufficient condition for hydrostatic equilibrium in general relativity
Authors: P. S. Negi
Comments: 31 pages (double-spaced) revtex style, 1 figure in `ps' format

We present explicit examples to show that the `compatibility criterion' is capable of providing a {\em necessary} and {\em sufficient} condition for any regular configuration to be compatible with the state of hydrostatic equilibrium. This conclusion is drawn on the basis of the finding that the $M-R$ relation gives the necessary and sufficient condition for dynamical stability of equilibrium configurations only when the compatibility criterion for these configurations is appropriately satisfied. In this regard, we construct an appropriate sequence composed of core-envelope models on the basis of compatibility criterion, such that each member of this sequence satisfies the extreme case of causality condition $v = c = 1$ at the centre. The maximum stable value of $u \simeq 0.3389$ (which occurs for the model corresponding to the maximum value of mass in the mass-radius relation) and the corresponding central value of the local adiabatic index, $(\Gamma_1)_0 \simeq 2.5911$, of this model are found fully consistent with those of the corresponding {\em absolute} values, $u_{\rm max} \leq 0.3406$, and $(\Gamma_1)_0 \leq 2.5946$, which impose strong constraints on these parameters of such models. In addition to this example, we also study dynamical stability of pure adiabatic polytropic configurations on the basis of variational method for the choice of the `trial function', $\xi =re^{\nu/4}$, as well as the mass-central density relation, since the compatibility criterion is appropriately satisfied for these models. The results of this example provide additional proof in favour of the statement regarding compatibility criterion mentioned above.

 

astro-ph/0606023 [abs, ps, pdf, other] :

Title: Deep Near-Infrared Imaging af an Embedded Cluster in the Extreme Outer Galaxy: Census of Supernovae Triggered Star Formation
Authors: Chikako Yasui, Naoto Kobayashi (IoA, University of Tokyo), Alan T. Tokunaga (IfA, University of Hawaii), Hiroshi Terada, Masao Saito (NAOJ)
Comments: 19pages, 8 figures, 1 table, accepted to ApJ. Full paper (pdf) with high resolution figures available at this http URL

While conducting a near-infrared (NIR) survey of ``Digel Clouds'', which are thought to be located in the extreme outer Galaxy (EOG), Kobayashi & Tokunaga found star formation activity in ``Cloud 2'', a giant molecular cloud at the Galactic radius of ~ 20 kpc. Additional infrared imaging showed two embedded young clusters at the densest regions of the molecular cloud. Because the molecular cloud is located in the vicinity of a supernova remnant (SNR) HI shell, GSH 138-01-94, it was suggested that the star formation activity in Cloud 2 was triggered by this expanding HI shell. We obtained deep J (1.25 um), H (1.65 um) and K (2.2 um) images of one of the embedded clusters in Cloud 2 with high spatial resolution (FWHM ~0".3) and high sensitivity (K ~ 20 mag, 10 sigma). We identified 52 cluster members. The estimated stellar density (~ 10 pc^{-2}) suggests that the cluster is a T-association. This is the deepest NIR imaging of an embedded cluster in the EOG. The observed K-band luminosity function (KLF) suggests that the underlying initial mass function (IMF) of the cluster down to the detection limit of ~ 0.1 M_sun is not significantly different from the typical IMFs in the field and in the near-by star clusters. The overall characteristics of this cluster appears to be similar to those of other embedded clusters in the far outer Galaxy. The estimated age of the cluster from the KLF, which is less than 1 Myr, is consistent with the view that the star formation was triggered by the HI shell whose age was estimated at 4.3 Myr (Stil & Irwin). The 3-dimensional geometry of SNR shell, molecular cloud and the embedded cluster, which is inferred from our data, as well as the cluster age strongly suggest that the star formation in Cloud 2 was triggered by the SNR shell.

 

astro-ph/0606024 [abs, ps, pdf, other] :

Title: Estimations of orbital parameters of exoplanets from transit photometry by using dynamical constraints
Authors: Zsolt Sandor
Comments: 16 pages, 10 figures, accepted for Cel. Mech. Dyn. Astron

The probability of the detection of Earth-like exoplanets may increase in the near future after the launch of the space missions using the transit photometry as observation method. By using this technique only the semi-major axis of the detected planet can be determined, and there will be no information on the upper limit of its orbital eccentricity. However, the orbital eccentricity is a very important parameter, not only from a dynamical point of view, since it gives also information on the climate and the habitability of the Earth-like planets. In this paper a possible procedure is suggested for confining the eccentricity of an exoplanet discovered by transit photometry if an already known giant planet orbits also in the system.

 

astro-ph/0606025 [abs, ps, pdf, other] :

Title: Instantaneous Radio Spectra of Giant Pulses from the Crab Pulsar from Decimeter to Decameter Wavelengths
Authors: M.V. Popov (ASC Lpi), A.D. Kuzmin (PRAO Asc Lpi), O.M. Ul'yanov (IRA, Kharkov), A.A. Deshpande (RRI, Bangalore; Arecibo), A.A. Ershov (PRAO Asc Lpi), V.V. Zakharenko (IRA, Kharkov), V.I. Kondratiev (ASC Lpi), S.V. Kostyuk (ASC Lpi), B.Ya. Losovskii (PRAO Asc Lpi), V.A. Soglasnov (ASC Lpi)
Comments: 13 pages, 1 figure, 1 table (originally published in Russian in Astronomicheskii Zhurnal, 2006, vol. 83, No. 7, pp. 630-637), translated by Georgii Rudnitskii
Journal-ref: Astron. Rep. 2006, vol. 50, No. 7, pp. 562-568

The results of simultaneous multifrequency observations of giant radio pulses from the Crab pulsar, PSR B0531+21, at 23, 111, and 600 MHz are presented and analyzed. Giant pulses were detected at a frequency as low as 23 MHz for the first time. Of the 45 giant pulses detected at 23 MHz, 12 were identified with counterparts observed simultaneously at 600 MHz. Of the 128 giant pulses detected at 111 MHz, 21 were identified with counterparts observed simultaneously at 600 MHz. The spectral indices for the power-law frequency dependence of the giant-pulse energies are from -3.1 to -1.6. The mean spectral index is -2.7 +/- 0.1 and is the same for both frequency combinations (600-111 MHz and 600-23 MHz). The large scatter in the spectral indices of the individual pulses and the large number of unidentified giant pulses suggest that the spectra of the individual giant pulses do not actually follow a simple power law. The observed shapes of the giant pulses at all three frequencies are determined by scattering on interstellar plasma irregularities. The scatter broadening of the pulses and its frequency dependence were determined as tau_sc=20*(f/100)^(-3.5 +/- 0.1) ms, where the frequency f is in MHz.

 

astro-ph/0606026 [abs, ps, pdf, other] :

Title: Hot Flashers and He Dwarfs in Galactic Globulars
Authors: M. Castellani, V. Castellani, P.G. Prada Moroni
Comments: 6 pages, 5 figures, accepted for publication on A&A

We revisit the evolutionary scenario for Hot Flasher low-mass structures, where mass loss delays the He flash till the initial phases of their White Dwarf cooling sequence. Our aim has been to test the theoretical results vis-a-vis different assumptions about the efficiency of mass loss. To this purpose, we present evolutionary models covering a fine grid of masses, as obtained assuming a single episode of mass loss in a Red Giant model of 0.86 Mo with Z=0.0015. We find a reasonable agreement with previous evolutionary investigations, showing that for the given metallicity late Hot Flashers are predicted to cover the mass range M=0.4975 to M= 0.4845 Mo, all models igniting the He-flash with a mass of the H-rich envelope as given by about Me=0.00050. The ignition mechanism is discussed in some details, showing the occurrence of a bifurcation in the evolutionary history of stellar structures at the lower mass limit for He ignition. Below such a critical mass, the structures miss the He ignition, cooling down as a Hot Flasher-Manque' He White Dwarfs We predict that these structures will cool down, reaching the luminosity logL/Lo=-1 in a time at the least five times longer than the corresponding cooling time of a normal CO White Dwarf. On very general grounds, one expects that old stellar clusters with a sizeable population of Hot Flasher should likely produce at least a similar amount of slow-cooling He White Dwarfs. According to this result, in a cluster where 20% of Red Giants escape the He burning phase, one expects roughly twice as White Dwarfs than in a normal cluster where all Red Giants undergo their He flash

 

astro-ph/0606027 [abs, ps, pdf, other] :

Title: Distances to Anomalous X-ray Pulsars using Red Clump Stars
Authors: Martin Durant, Marten H. van Kerkwijk (University of Toronto)
Comments: 38 pages aastex, 10 figures. Accepted for publication in ApJ

We identify `red clump stars' - core helium-burning giants - among 2MASS stars and use them to measure the run of reddening with distance in the direction of each of the Galactic Anomalous X-ray Pulsars (AXP). We combine this with extinction estimates from X-ray spectroscopy to infer distances and find that the locations of all AXP are consistent with being in Galactic spiral arms. We also find that the 2-10 keV luminosities implied by our distances are remarkably similar for all AXPs, being all around 1.3e35 erg s-1. Furthermore, using our distances to estimate effective black-body emitting radii, we find that the radii are tightly anti-correlated with pulsed fraction, and somewhat less tightly anti-correlated with black-body temperature. We find no obvious relationship of any property with the dipole magnetic field strength inferred from the spin-down rate.

 

astro-ph/0606028 [abs, ps, pdf, other] :

Title: Dark matter clustering: a simple renormalization group approach
Authors: Patrick McDonald (CITA)
Comments: 24 pages, 8 figures, to be submitted to PRD

I compute a renormalization group (RG) improvement to the standard beyond-linear-order Eulerian perturbation theory (PT) calculation of the power spectrum of large-scale density fluctuations in the Universe. At z=0, for a power spectrum matching current observations, lowest order RGPT appears to be as accurate as one can test using existing numerical simulation-calibrated fitting formulas out to at least k~=0.3 h/Mpc. In contrast, standard PT breaks down virtually as soon as beyond-linear corrections become non-negligible, at k<0.1 h/Mpc. This extension in range of validity could substantially enhance the usefulness of PT for interpreting baryonic acoustic oscillation surveys aimed at probing dark energy, for example. I show that the predicted power spectrum converges at high k to a power law with index given by the fixed-point solution of the RG equation. I discuss many possible future directions for this line of work. The paper should be easily readable without any prior knowledge of RG methods.

 

astro-ph/0606029 [abs, ps, pdf, other] :

Title: On the effect of overshooting as predicted by the modelling of the pre-main sequence evolution of a 2 solar-mass star
Authors: J P Marques, M J P F G Monteiro, J Fernandes
Comments: 10 pages and 7 figures - to appear in MNRAS

We discuss the effects of convective overshooting in the PMS evolution of intermediate mass stars, by analysing in detail the early evolution towards the main sequence of a 2 M_sun stellar model. These effects can be extremely important in the end of the PMS, when the abundances in CNO elements approach the equilibrium in the centre. We provide a possible physical explanation on why a moderate amount of overshooting produces, as the star approaches the ZAMS, an extra loop in the evolutionary tracks on the HR diagram.
An interesting feature is that there is a very well defined amount of overshooting (for a given stellar mass and chemical composition) beyond which a loop is produced. For smaller amounts of overshooting such a loop does not take place and the evolutionary tracks are similar to the ones obtained by Iben (1965). The amount of overshooting needed to produce the loop decreases with stellar mass.
We discuss the underlining physical reasons for the behaviour predicted by the evolution models and argue that it provides a crucial observational test for convective overshooting in the core of intermediate mass stars.

 

astro-ph/0606030 [abs, ps, pdf, other] :

Title: GRB 050315: A step toward the uniqueness of the overall GRB structure
Authors: Remo Ruffini, Maria Grazia Bernardini, Carlo Luciano Bianco, Pascal Chardonnet, Federico Fraschetti, Roberto Guida, She-Sheng Xue
Comments: 4 pages, 3 figures

Using the Swift data of GRB 050315, we progress on the uniqueness of our theoretically predicted Gamma-Ray Burst (GRB) structure as composed by a proper-GRB (P-GRB), emitted at the transparency of an electron-positron plasma with suitable baryon loading, and an afterglow comprising the so called "prompt emission" as due to external shocks. Thanks to the Swift observations, the P-GRB is identified and for the first time we can theoretically fit detailed light curves for selected energy bands on a continuous time scale ranging over 10^6 seconds. The theoretically predicted instantaneous spectral distribution over the entire afterglow is presented, confirming a clear hard-to-soft behavior encompassing, continuously, the "prompt emission" all the way to the latest phases of the afterglow.

 

astro-ph/0606031 [abs, pdf] :

Title: Simulating Anisotropic Thermal Conduction in Supernova Remnants I : Numerics and the Evolution of Remnants
Authors: D. S. Balsara, D. A. Tilley, J. C. Howk

Anisotropic thermal conduction plays an important role in various astrophysical systems. One of the most stringent tests of thermal conduction can be found in supernova remnants. In this paper we study anisotropic thermal conduction and examine the physical nature of the flux of thermal conduction in the classical and saturated limits. We also present a temporally second-order accurate implicit-explicit scheme for the time-update of thermal conduction terms within a numerical MHD scheme.
Several simulations of supernova remnants are presented for a range of ISM parameters. The role of thermal conduction in such remnants has been studied. We find that thermal conduction produces cooler temperatures and higher densities in the hot gas bubbles that form in the remnants. The effect of thermal conduction in changing the thermal characteristics of the hot gas bubble increases as the remnant propagates through denser ISMs. Remnants evolving in denser ISMs are shown to make a faster transition to a centre-bright x-ray morphology, with the trend emerging earlier in hard x-rays than in the soft x-rays.

 

astro-ph/0606032 [abs, ps, pdf, other] :

Title: A Cloudy/Xspec Interface
Authors: R. L. Porter (1), G. J. Ferland (1), S. B. Kraemer (2), B. K. Armentrout (2), K. A. Arnaud (3), T. J. Turner (4) ((1) Dept. of Physics and Astronomy, University of Kentucky, (2) Dept. of Physics, Catholic University of America, (3) Laboratory for X-ray Astrophysics, NASA GSFC, (4) Joint Center for Astrophysics, Physics Department, University of Maryland)
Comments: 13 pages, 1 table, 4 figures, accepted for publication in PASP

We discuss new functionality of the spectral simulation code CLOUDY which allows the user to calculate grids with one or more initial parameters varied and formats the predicted spectra in the standard FITS format. These files can then be imported into the x-ray spectral analysis software XSPEC and used as theoretical models for observations. We present and verify a test case. Finally, we consider a few observations and discuss our results.

 

astro-ph/0606033 [abs, ps, pdf, other] :

Title: Inflation with large gravitational waves
Authors: Alexander Vikman
Comments: Contributed to XLIst Rencontres de Moriond Workshop on Contents and Structures of the Universe, La Thuile, Italy, March 18-25, 2006

It is well known that in manifestly Lorentz invariant theories with nontrivial kinetic terms, perturbations around some classical backgrounds can travel faster than light. These exotic "supersonic" models may have interesting consequences for cosmology and astrophysics. In particular, one can show that in such theories the contribution of the gravitational waves to the CMB fluctuations can be significantly larger than that in standard inflationary models. This increase of the tensor-to-scalar perturbation ratio leads to a larger B-component of the CMB polarization, thus making the prospects for future detection much more promising. Interestingly, the spectral index of scalar perturbations and mass of the scalar field considered in the model are practically indistinguishable from the standard case. Whereas the energy scale of inflation and hence the reheating temperature can be much higher compared to a simple chaotic inflation.

 

astro-ph/0606034 [abs, ps, pdf, other] :

Title: Stability in MaVaN Models
Authors: Christopher Spitzer
Comments: 5 pages, 3 figures

Mass-varying neutrino (MaVaN) models propose a source of dark energy in a new scalar field called the acceleron. Recent work has shown that nonrelativistic neutrino fields in these theories are unstable to inhomogeneous fluctuations, and form structures that no longer behave as dark energy. One might expect that in multiple-neutrino models, the lighter species could continue to act as a source for the acceleron, generating dark energy without the help of heavier species. This paper shows that by considering the evolution of the acceleron field for a large class of models, the result of any neutrino component becoming unstable is that all components become unstable within a short time on cosmological scales. An alternate model employing a second scalar field in a hybrid potential is shown to have stable MaVaN dark energy even in the presence of unstable heavier components.