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Astrophysics

astro-ph new abstracts, Tue, 10 Jan 06 01:00:10 GMT
0601138 -- 0601191 received

astro-ph/0601138 [abs, ps, pdf, other] :

Title: The Bar-Halo Interaction - II. Secular evolution and the religion of N-body simulations
Authors: Martin D. Weinberg, Neal Katz
Comments: 23 pages, 18 figures, submitted to Monthly Notices. For paper with figures at full resolution: this http URL

This paper explores resonance-driven secular evolution between a bar and dark-matter halo using N-body simulations. We make direct comparisons to our analytic theory (Weinberg & Katz 2005) to demonstrate the great difficulty that an N-body simulation has representing these dynamics for realistic astronomical interactions. In a dark-matter halo, the bar's angular momentum is coupled to the central density cusp (if present) by the Inner Lindblad Resonance. Owing to this angular momentum transfer and self-consistent re-equilibration, strong realistic bars WILL modify the cusp profile, lowering the central densities within about 30% of the bar radius in a few bar orbits. Past results to the contrary (Sellwood 2006, McMillan & Dehnen 2005) may be the result of weak bars or numerical artifacts. The magnitude depends on many factors and we illustrate the sensitivity of the response to the dark-matter profile, the bar shape and mass, and the galaxy's evolutionary history. For example, if the bar length is comparable to the size of a central dark-matter core, the bar may exchange angular momentum without changing its pattern speed significantly. We emphasise that this apparently simple example of secular evolution is remarkably subtle in detail and conclude that an N-body exploration of any astronomical scenario requires a deep investigation into the underlying dynamical mechanisms for that particular problem to set the necessary requirements for the simulation parameters and method (e.g. particle number and Poisson solver). Simply put, N-body simulations do not divinely reveal truth and hence their results are not infallible. They are unlikely to provide useful insight on their own, particularly for the study of even more complex secular processes such as the production of pseudo-bulges and disk heating.

 

astro-ph/0601139 [abs, ps, pdf, other] :

Title: The Galactic Nature of High Velocity Cloud Complex WB
Authors: C. Thom (Swinburne), M. E. Putman (UMich), B. K. Gibson (Swinburne), N. Christlieb (Hamburg), C. Flynn (Tuorla Obs), T. C. Beers (MSU), R. Wilhelm (Texas Tech), Y.S. Lee (MSU)
Comments: 2 Figures, 1 Table. ApJL accepted

We have detected absorption lines from the High Velocity Cloud Complex WB in the spectrum of the star HE1048+0231. This detection sets an upper distance limit to the cloud of 8.8^{+2.3}_{-1.3} kpc. Non-detection (at >4 sigma confidence) in the star HE1138-1303 at 7.7 +/- 0.2 kpc sets a probable lower limit. The equivalent width of the CaII K line due to the HVC (114.6 +/- 4.4 mA) corresponds to a column density of 1.32 +/- 0.05 x 10^12 cm^-2. Using an HI spectrum from the Leiden/Argentine/Bonn survey, we calculated N(CaII)/N(HI) = 81 +/- 16 x 10^-9. These distance limits imply an HI mass limit of 3.8 x 10^5 Msun < M_HI < 4.9 x 10^5 Msun. The upper distance limit imposed by these observations shows that this HVC complex has a probable Galactic or circum-Galactic origin. Future metallicity measurements will be able to confirm or refute this interpretation.

 

astro-ph/0601140 [abs, ps, pdf, other] :

Title: Gravitational microlensing of quasar Broad Line Regions: the influence of fractal structures
Authors: Geraint F. Lewis, Rodrigo A. Ibata
Comments: 6 pages, accepted for publication in MNRAS (note, resolution of figures reduced to size restrictions)

Recent models for the emission clouds within the Broad Line Region of quasars suggest that they are due to transient overdensities within an overall turbulent medium. If this were the case, the broad line emission would spatially appear fractal, possessing structure on a range of scales. This paper examines the influence of such fractal structure when a quasar is microlensed by a population of intervening masses. It is found that while the highest fractal levels can undergo significant microlensing magnification, when these light curves are superimposed to create an emission line profile, the resultant emission line profile remains relatively constant for physical models of the Broad Line Region. It is concluded that the detection of the possible fractal structure of Broad Line Regions via gravitational microlensing is not practical.

 

astro-ph/0601141 [abs, ps, pdf, other] :

Title: The X-ray properties of young radio-loud AGN
Authors: Jacco Vink (1), Ignas Snellen (2), Karl-Heinz Mack (3), Richard Schilizzi (4) ((1) Utrecht University/SRON Netherlands Institute for Space research, (2) Leiden Observatory, (3) INAF - Istituto di Radioastronomia, Bologna,(4) International SKA Project Office, Dwingeloo)
Comments: 11 pages, 6 figures. Accepted for publication by the MNRAS

We present XMM-Newton observations of a complete sample of five archetypal young radio-loud AGN, also known Gigahertz Peaked Spectrum (GPS) sources. They are among the brightest and best studied GPS/CSO sources in the sky, with radio powers in the range L_{5GHz}=10^{43-44} erg/s and with 4 sources having measured kinematic ages of 570 to 3000 yrs. All sources are detected, and have 2-10 keV luminosities from 0.5 to 4.8x10^{44} erg/s. In comparison with the general population of radio galaxies, we find that: 1) GPS galaxies show a a range in absorption column densities similar to other radio galaxies. We therefore find no evidence that GPS galaxies reside in significantly more dense circumnuclear environment, such that they could be hampered in their expansion. 2) The ratio of radio to X-ray luminosity is significantly higher than for classical radio sources. This is consistent with an evolution scenario in which young radio sources are more efficient radio emitters than large extended objects at a constant accretion power. 3) Taking the X-ray luminosity of radio sources as a measure of ionisation power, we find that GPS galaxies are significantly underluminous in their [OIII]_{5007 Angstrom}, including a weak trend with age. This is consistent with the fact that the Stroemgren sphere should still be expanding in these young objects. This would mean that here we are witnessing the birth of the narrow line region of radio-loud AGN.

 

astro-ph/0601142 [abs, ps, pdf, other] :

Title: Collapse of Neutron Stars to Black Holes in Binary Systems: A Model for Short Gamma Ray Bursts
Authors: Charles D. Dermer (1), Armen Atoyan (2) ((1) NRL, (2) University of Montreal)
Comments: 9 pages, submitted as a letter to the Astrophysical Journal

The accretion of ~ 0.1 -- 1 M_o of material by a neutron star through Roche lobe overflow of its companion or through white-dwarf/neutron-star coalescence in a low mass binary system could be enough to exceed the critical mass of a neutron star and trigger its collapse to a black hole, leading to the production of a short gamma-ray burst (SGRB). In this model, SGRBs would often be found in early-type galaxies or in globular cluster environments, though they could also be formed through stellar-wind accretion in high-mass binary systems of star-forming galaxies. Although the collapse event is likely to be of short (<< 1 s) duration, afterglow complexity could be produced by neutron decoupling in the fireball and subsequent accretion of the companion remnant, for example, the disrupted white-dwarf core.

 

astro-ph/0601143 [abs, ps, pdf, other] :

Title: Radiative Flow in a Luminous Disk
Authors: Jun Fukue
Comments: 7 pages, 4 figures
Journal-ref: PASJ 57 (2005) 841-847

Radiatively-driven flow in a luminous disk is examined in the subrelativistic regime of $(v/c)^1$, taking account of radiation transfer. The flow is assumed to be vertical, and the gravity and gas pressure are ignored. When internal heating is dropped, for a given optical depth and radiation pressure at the flow base (disk ``inside''), where the flow speed is zero, the flow is analytically solved under the appropriate boundary condition at the flow top (disk ``surface''), where the optical depth is zero. The loaded mass and terminal speed of the flow are both determined by the initial conditions; the mass-loss rate increases as the initial radiation pressure increases, while the flow terminal speed increases as the initial radiation pressure and the loaded mass decrease. In particular, when heating is ignored, the radiative flux $F$ is constant, and the radiation pressure $P_0$ at the flow base with optical depth $\tau_0$ is bound in the range of $2/3 < cP_0/F < 2/3 + \tau_0$. In this case, in the limit of $cP_0/F = 2/3 + \tau_0$, the loaded mass diverges and the flow terminal speed becomes zero, while, in the limit of $cP_0/F = 2/3$, the loaded mass becomes zero and the terminal speed approaches $(3/8)c$, which is the terminal speed above the luminous flat disk under an approximation of the order of $(v/c)^1$. We also examine the case where heating exists, and find that the flow properties are qualitatively similar to the case without heating.

 

astro-ph/0601144 [abs, ps, pdf, other] :

Title: Radiative Flow in a Luminous Disk II
Authors: Jun Fukue
Comments: 6 pages, 4 figures; to be appeared in PASJ 58 (2006) No 1

Radiatively-driven transfer flow perpendicular to a luminous disk is examined in the subrelativistic regime of $(v/c)^1$, taking into account the gravity of the central object. The flow is assumed to be vertical, and the gas pressure is ignored, while internal heating is assumed to be proportional to the gas density. The basic equations were numerically solved as a function of the optical depth, and the flow velocity, the height, the radiative flux, and the radiation pressure were obtained for a given radius, an initial optical depth, and initial conditions at the flow base (disk ``inside''), whereas the mass-loss rate was determined as an eigenvalue of the boundary condition at the flow top (disk ``surface''). For sufficiently luminous cases, the flow resembles the case without gravity. For less-luminous cases, however, the flow velocity decreases, and the flow would be impossible due to the existence of gravity in the case that the radiative flux is sufficiently small. Application to a supercritical accretion disk with mass loss is briefly discussed.

 

astro-ph/0601145 [abs, ps, pdf, other] :

Title: Novel Methods for Predicting Photometric Redshifts from Broad Band Photometry using Virtual Sensors
Authors: M.J. Way, A.N. Srivastava (NASA/Ames Research Center)
Comments: 32 pages, 26 figures, Submitted to ApJ

We calculate photometric redshifts from the Sloan Digital Sky Survey Main Galaxy Sample, The Galaxy Evolution Explorer All Sky Survey, and The Two Micron All Sky Survey using two new training-set methods. We utilize the broad-band photometry from the three surveys alongside Sloan Digital Sky Survey measures of photometric quality and galaxy morphology. Our first training-set method draws from the theory of ensemble learning while the second employs Gaussian process regression both of which allow for the estimation of redshift along with a measure of uncertainty in the estimation. The Gaussian process models the data very effectively with small training samples of approximately 1000 points or less. These two methods are compared to a well known Artificial Neural Network training-set method and to simple linear and quadratic regression. We also demonstrate the need to provide confidence bands on the error estimation made by both classes of models. Our results indicate that variations due to the optimization procedure used for almost all neural networks, combined with the variations due to the data sample, can produce models with variations in accuracy that span an order of magnitude. A key contribution of this paper is to quantify the variability in the quality of results as a function of model and training sample. We show how simply choosing the "best" model given a data set and model class can produce misleading results. (abridged)

 

astro-ph/0601146 [abs, ps, pdf, other] :

Title: Quantifying the Luminosity Evolution in Gamma-ray Bursts
Authors: Daniel Kocevski, Edison Liang

We estimate the luminosity evolution and formation rate for over 900 GRBs by using redshift and luminosity data calculated by Band, Norris, $&$ Bonnell (2004) via the lag-luminosity correlation. By applying maximum likelihood techniques, we are able to infer the true distribution of the parent GRB population's luminosity function and density distributions in a way that accounts for detector selection effects. We find that after accounting for data truncation, there still exists a significant correlation between the average luminosity and redshift, indicating that distant GRBs are on average more luminous than nearby counterparts. This is consistent with previous studies showing strong source evolution and also recent observations of under luminous nearby GRBs. We find no evidence for beaming angle evolution in the current sample of GRBs with known redshift, suggesting that this increase in luminosity can not be due to an evolution of the collimation of gamma-ray emission. The resulting luminosity function is well fit with a single power law of index $L'^{-1.5}$, which is intermediate between the values predicted by the power-law and Gaussian structured jet models. We also find that the GRB comoving rate density rises steeply with a broad peak between $1<z<2$ followed by a steady decline above $z> 3$. This rate density qualitatively matches the current estimates of the cosmic star formation rate, favoring a short lived massive star progenitor model, or a binary model with a short delay between the formation of the compact object and the eventual merger.

 

astro-ph/0601147 [abs, ps, pdf, other] :

Title: Discovery of a 500 pc shell in the nucleus of Centaurus A
Authors: Alice C. Quillen (Rochester), Joss Bland-Hawthorn (AAO), Mairi H. Brookes (JPL), Michael W. Werner (JPL), J. D. Smith (Steward Obs), Daniel Stern (JPL), Jocelyn Keene (JPL), Charles R. Lawrence (JPL)
Comments: submitted to ApJ Letters

Spitzer Space Telescope mid-infrared images of the radio galaxy Centaurus A reveal a shell-like, bipolar, structure 500 pc to the north and south of the nucleus. This shell is seen in 5.8, 8.0 and 24 micron broad-band images. Such a remarkable shell has not been previously detected in a radio galaxy and is the first extragalactic nuclear shell detected at mid-infrared wavelengths. We estimate that the shell is a few million years old and has a mass of order million solar masses. A conservative estimate for the mechanical energy in the wind driven bubble is 10^53 erg. The shell could have created by a small few thousand solar mass nuclear burst of star formation. Alternatively, the bolometric luminosity of the active nucleus is sufficiently large that it could power the shell. Constraints on the shell's velocity are lacking. However, if the shell is moving at 1000 km/s then the required mechanical energy would be 100 times larger.

 

astro-ph/0601148 [abs, ps, pdf, other] :

Title: RICE Limits on the Diffuse Ultra-High Energy Neutrino Flux
Authors: I. Kravchenko, et al
Comments: Submitted to Astropart. Phys

We present new limits on ultra-high energy neutrino fluxes above 100 PeV based on data collected by the Radio Ice Cherenkov Experiment (RICE) at the South Pole from 1999-2005. We discuss estimation of backgrounds, calibration and data analysis algorithms (both on-line and off-line), procedures used for the dedicated neutrino search, and refinements in our Monte Carlo (MC) simulation, including recent in situ measurements of the complex ice dielectric constant. An enlarged data set and a more detailed study of hadronic showers results in a sensitivity improvement of more than one order of magnitude compared to our previously published results. Examination of the full RICE data set yields zero acceptable neutrino candidates, resulting in 95% confidence-level model dependent limits on the flux (E_\nu)^2(d\phi/dE_\nu)<10^{-6} GeV/(cm^2s~sr}) in the energy range 10^{17}< E_\nu< 10^{20} eV. The new RICE results rule out the most intense flux model projections at 95% confidence level.

 

astro-ph/0601149 [abs, ps, pdf, other] :

Title: Time Delays for the Giant Quadruple Lensed Quasar SDSS J1004+4112: Prospects for Determining the Density Profile of the Lensing Cluster
Authors: Yozo Kawano (Nagoya U.), Masamune Oguri (Princeton U.)
Comments: 21 pages, 8 figures, accepted for publication in PASJ

We investigate the dependence of the time delays for the large-separation gravitationally lensed quasar SDSS J1004+4112 on the inner mass profile of the lensing cluster. Adopting the mass model whose innermost density profile is parameterized as \rho \propto r^{-\alpha}, we derive a series of mass models which can fit observational data and then compute the probability distribution functions of time delays. We find that larger \alpha has longer time delays, longer tails at the higher end of the probability distribution, and larger model uncertainties. The ratios of time delays slightly depend on the slope \alpha. Among others, time delays between images C and A (or B) have little dependence on the inner slope, particularly when the time delays are short. The dependence of time delays on \alpha is well fitted by a linear form, which reflects well-known degeneracy between the mass profile and time delays. We perform a Monte-Carlo simulation to illustrate how well the inner slope can be constrained from measurements of time delays. We find that measurements of more than one time delays result in reasonably tight constraints on the inner slope (\sigma_{\alpha} \lesssim 0.25), while only one time delay cannot determine the inner slope very well. Our result indicates that time delays indeed serve as a powerful tool to determine the mass profile, despite the complexity of the lensing cluster.

 

astro-ph/0601150 [abs, ps, pdf, other] :

Title: An improved redshift indicator for Gamma-Ray Bursts, based on the prompt emission
Authors: Alexandre Pélangeon (LAOMP), Jean-Luc Atteia (LAOMP), Don Q. Lamb, George R. Ricker, the the HETE Science Team Collaboration
Comments: To appear in the proceedings of the 16th Annual October Astrophysics Conference in Maryland, "Gamma Ray Bursts in the Swift Era", Washington DC., November 29-December 2, 2005, 4 pages, 3 figures

We propose an improved version of the redshift indicator developed by Atteia (2003), which gets rid of the dependence on the burst duration and provides better estimates for high-redshift GRBs. We present the derivation and the definition of this redshift indicator, then its calibration with 17 GRBs with known redshifts detected by HETE-2 and 2 more detected by Konus-Wind. We also provide an estimation of the redshift for 59 bursts, and we finally discuss the redshift distribution of HETE-bursts and the possible other applications of this redshift indicator.

 

astro-ph/0601151 [abs, ps, pdf, other] :

Title: Spectral analysis of extinguished sunlight
Authors: Frederic Zagury, Florence Goutail
Comments: 6 figures
Journal-ref: New Astronomy, 2003, 8, p.537-548

SAOZ (Systeme d'Analyse par Observation Zenitale) is a balloon born experiment which determines the column density of several molecular species from the visible spectrum of sunlight. We will use sequence of spectra collected during a sunset to discuss atmospheric extinction, and the nature of the radiation field in the atmosphere. The radiation field in the atmosphere is, from daylight to sunset, and with a clear sky, dominated by light coming from the direction of the sun. This light is composed of direct sunlight (extinguished by the gas), and of sunlight forward-scattered by aerosols. As the sun sets, aerosol scattering is first perceived towards the UV. It progressively replaces direct sunlight over all of the spectrum. Our analysis permits fixing the main parameters of each component of the radiation field at any time. The fits we find for the extinction of sunlight in the atmosphere must also apply to starlight. Thus, the present work can be used in astronomy to correct ground-based spectral observations for extinction in the atmosphere.

 

astro-ph/0601152 [abs, ps, pdf, other] :

Title: Spectral analysis of red scattered sunlight at sunrise
Authors: Frederic Zagury, Mitsugu Fujii
Comments: 5 figures
Journal-ref: New Astronomy, 2003, 8, p.549-556

We analyze and fit visible spectra of a red horizon at sunrise. The shape of the spectra consist of a blue continuum followed by a red bump. The reddest spectra are well fitted by the product of a spectrum of extinguished sunlight (Rayleigh extinction + ozone absorption) and 1/lambda^4. The former is essentially the radiation field in the outer atmosphere, at the scattering volume location; the latter corresponds to Rayleigh scattering by the gas. Moving to higher altitudes, a second component, corresponding to the spectrum of a blue sky, must be added. The spectra we have obtained are similar to spectra of red nebulae, suggesting there may be other explanations than an emission process to the red color of some nebulae.

 

astro-ph/0601153 [abs, ps, pdf, other] :

Title: Inflation and accelerated expansion TeVeS cosmological solutions
Authors: Luz Maria Diaz-Rivera, Lado Samushia, Bharat Ratra

We find exact exponentially expanding and contracting de Sitter solutions of the spatially homogeneous TeVeS cosmological equations of motion, in both the vacuum TeVeS model and in the presence of an additional ideal fluid with equation of state parameter $\omega < -1$. A preliminary stability analysis shows that the expanding vacuum solution is stable, while in the ideal fluid case stability depends on model parameter values. These solutions might provide a basis for incorporating early-time inflation or late-time accelerated expansion in TeVeS cosmology.

 

astro-ph/0601154 [abs, ps, pdf, other] :

Title: Excitation of g modes in Wolf-Rayet stars by a deep opacity bump
Authors: R H D Townsend, J MacDonald
Comments: 5 pages, 3 figures, submitted to MNRAS letters

We examine the stability of l=1 and l=2 g modes in a pair of nitrogen-rich Wolf-Rayet stellar models characterized by differing hydrogen abundances. We find that modes with intermediate radial orders are destabilized by a kappa mechanism operating on an opacity bump at an envelope temperature log T ~ 6.25. This `deep opacity bump' is due primarily to L-shell bound-free transitions of iron. Periods of the unstable modes span ~ 11-21 hr in the model containing some hydrogen, and ~ 3-12 hr in the hydrogen-depleted model. Based on the latter finding, we suggest that self-excited g modes may be the source of the 9.8 hr-periodic variation of WR 123 recently reported by Lefevre et al. (2005).

 

astro-ph/0601155 [abs, ps, pdf, other] :

Title: Detectability of CII 158 micron emission from high-redshift galaxies: predictions for ALMA and SPICA
Authors: Kentaro Nagamine (1), Arthur M. Wolfe (1), Lars Hernquist (2) ((1) UCSD, (2) Harvard)
Comments: 37 pages, 12 figures, submitted to ApJ

We discuss the detectability of high-redshift galaxies via CII 158 micron line emission by coupling an analytic model with cosmological Smoothed Particle Hydrodynamics (SPH) simulations that are based on the concordance Lambda cold dark matter (CDM) model. Our analytic model describes a multiphase interstellar medium irradiated by the far ultra-violet radiation from local star-forming regions, and it calculates thermal and ionization equilibrium between cooling and heating. The model allows us to predict the mass fraction of a cold neutral medium (CNM) embedded in a warm neutral medium (WNM). Our cosmological SPH simulations include a treatment of radiative cooling/heating, star formation, and feedback effects from supernovae and galactic winds. Using our method, we make predictions for the CII luminosity from high-redshift galaxies which can be directly compared with upcoming observations by the Atacama Large Millimeter Array (ALMA) and the Space Infrared Telescope for Cosmology and Astrophysics (SPICA). We find that the number density of galaxies that are detectable by ALMA and SPICA via CII emission depends significantly on the amount of neutral gas in high-redshift galaxies, which is highly uncertain. In a CDM universe, most CII sources at z=3 are expected to be faint objects with \Snu < 0.01 mJy. Our calculations suggest that Lyman-break galaxies (LBGs) brighter than R_AB=23.5 magnitude have flux densities \Snu = 1-3 mJy depending on the strength of galactic wind feedback, and one has to aim at very bright LBGs to obtain a detection with ALMA and SPICA.

 

astro-ph/0601156 [abs, ps, pdf, other] :

Title: Mechanical Model for Relativistic Blast Waves and Stratified Fireballs
Authors: Z. Uhm, A. M. Beloborodov (Columbia University)
Comments: to be published in AIP Conference Proceedings "Gamma Ray Bursts in the Swift Era," Nov. 29 - Dec. 2, 2005, Washington, DC

We propose a simple mechanical model for relativistic explosions with both forward and reverse shocks, which allows one to do fast calculations of GRB afterglow. The blast wave in the model is governed by pressures P_F and P_R at the forward and reverse shocks. We show that the simplest assumption P_F=P_R is in general inconsistent with energy conservation law. The model is applied to GRBs with non-uniform ejecta. Such "stratified fireballs" are likely to emerge with a monotonic velocity profile after an internal-shock stage. We calculate the early afterglow emission expected from stratified fireballs.

 

astro-ph/0601157 [abs, ps, pdf, other] :

Title: Neutrino-cooled Accretion Disks around Spinning Black Holes
Authors: Wen-xin Chen, Andrei M. Beloborodov (Columbia University)
Comments: 4 pages, 3 figures; to be published in AIP Conference Proceedings "Gamma Ray Bursts in the Swift Era," Nov. 29 - Dec. 2, 2005, Washington, DC

We calculate the structure of accretion disk around a spinning black hole for accretion rates 0.01 - 10 M_sun/s. The model is fully relativistic and treats accurately the disk microphysics including neutrino emissivity, opacity, electron degeneracy, and nuclear composition. We find that the accretion flow always regulates itself to a mildly degenerate state with the proton-to-nucleon ratio Y_e ~ 0.1 and becomes very neutron-rich. The disk has a well defined "ignition" radius where neutrino flux raises dramatically, cooling becomes efficient, and Y_e suddenly drops. We also calculate other characteristic radii of the disk, including the neutrino-opaque and neutrino-trapping radii, and show their dependence on the accretion rate. Accretion disks around fast-rotating black holes produce intense neutrino fluxes which may deposit enough energy above the disk to generate a GRB jet.

 

astro-ph/0601158 [abs, ps, pdf, other] :

Title: Cosmic ray recipes
Authors: Franco Ferrari, Ewa Szuszkiewicz
Comments: 32 pages, 8 figures in eps format, plain LaTeX + package graphicx

Cosmic rays represent one of the most fascinating research themes in modern astronomy and physics. After almost a century since their discovery, a huge amount of scientific literature has been written on this topic and it is not always easy to extract from it the necessary information for somebody who approaches the subject for the first time. This has been the main motivation for preparing this article, which is a concise and self-contained review for whoever is interested in studying cosmic rays. The priority has been given here to well established facts, which are not at risk to get obsolete in a few years due to the fast progress of the research in this field. Also many data are presented, which are useful to characterize the doses of ionizing radiation delivered to organisms living on the Earth due to cosmic rays. The technical terms which are often encountered in the scientific literature are explained in a separate appendix.

 

astro-ph/0601159 [abs, ps, pdf, other] :

Title: An empirical calibration of the mixing-length parameter alpha
Authors: F.R. Ferraro, E. Valenti, O. Straniero, L. Origlia
Comments: 15 pages, 3 figures, ApJ in press

We present an empirical calibration of the Mixing-Length free parameter ${\alpha}$ based on a homogeneous Infrared database of 28 Galactic globular clusters spanning a wide metallicity range (-2.15$<$[Fe/H]$<$-0.2). Empirical estimates of the red giant effective temperatures have been obtained from infrared colors. Suitable relations linking these temperatures to the cluster metallicity have been obtained and compared to theoretical predictions. An appropriate set of models for the Sun and Population II giants have been computed by using both the standard solar metallicity $Z_{\odot}=0.0275$ and the most recently proposed value $Z_{\odot}=$0.0177. We find that when the standard solar metallicity is adopted, a unique value of $\alpha$=2.17 can be used to reproduce both the solar radius and the population II red giant temperature. Conversely, when the new solar metallicity is adopted, two different values of $\alpha$ are required: $\alpha=1.86$ to fit the solar radius and $\alpha \approx2.0$ to fit the red giant temperatures. However, it must be noted that, regardless the adopted solar reference, the $\alpha$ parameter does not show any significant dependence on metallicity.

 

astro-ph/0601160 [abs, ps, pdf, other] :

Title: Self-consistent response of a galactic disc to vertical perturbations
Authors: Kanak Saha, Chanda J. Jog
Comments: 10 pages, 5 figures, accepted for publication in MNRAS

We study the self-consistent, linear response of a galactic disc to vertical perturbations as induced say by a tidal interaction. We calculate the self-gravitational potential corresponding to a non-axisymmetric, self-consistent density response of the disc using the Green's function method. The response potential is shown to oppose the perturbation potential because the self-gravity of the disc resists the imposed potential, and this resistence is stronger in the inner parts of a galactic disc. For the m=1 azimuthal wavenumber, the disc response opposes the imposed perturbation upto a radius that spans a range of 4-6 disc scale-lengths, so that the disc shows a net warp only beyond this region. This physically explains the well-known but so far unexplained observation (Briggs 1990) that warps typically set in beyond this range of radii. We show that the inclusion of a dark matter halo in the calculation only marginally changes (by ~ 10 %) the radius for the onset of warps. For perturbations with higher azimuthal wavenumbers, the net signature of the vertical perturbations can only be seen at larger radii - for example beyond 7 exponential disc scale-lengths for m=10. Also, for high m cases, the magnitude of the negative disc response due to the disc self-gravity is much smaller. This is shown to result in corrugations of the mid-plane density, which explains the puzzling scalloping with m=10 detected in HI in the outermost regions ~ 30 kpc in the Galaxy by Kulkarni et al. (1982).

 

astro-ph/0601161 [abs, ps, pdf, other] :

Title: Resonant relaxation near a massive black hole: the stellar distribution and gravitational wave sources
Authors: Clovis Hopman, Tal Alexander (Weizmann)
Comments: Submitted to ApJ

Resonant relaxation (RR) of orbital angular momenta occurs near massive black holes (MBHs) where the stellar orbits are nearly Keplerian and so do not precess significantly. The resulting coherent torques efficiently change the magnitude of the angular momenta and rotate the orbital inclination in all directions. As a result, many of the tightly bound stars very near the MBH are rapidly destroyed by falling into the MBH on low-angular momentum orbits, while the orbits of the remaining stars are efficiently randomized. We solve numerically the Fokker-Planck equation in energy for the steady state distribution of a single mass population with a RR sink term. We find that the steady state current of stars, which sustains the accelerated drainage close to the MBH, can be up to ~10 times larger than that due to non-coherent 2-body relaxation alone. RR mostly affects tightly bound stars, and so it increases only moderately the total tidal disruption rate, which is dominated by stars originating from less bound orbits farther away. We show that the event rate of gravitational wave (GW) emission from inspiraling stars, originating much closer to the MBH, is dominated by RR dynamics. The GW event rate depends on the uncertain efficiency of RR. The efficiency indicated by the few available simulations implies rates ~10 times higher than those predicted by 2-body relaxation, which would improve the prospects of detecting such events by future GW detectors, such as LISA. However, a higher, but still plausible RR efficiency can lead to the drainage of all tightly bound stars and strong suppression of GW events from inspiraling stars. We apply our results to the Galactic MBH, and show that the observed dynamical properties of stars there are consistent with RR.

 

astro-ph/0601162 [abs, ps, pdf, other] :

Title: Optical Properties of Radio-selected Narrow Line Seyfert 1 Galaxies
Authors: D. J. Whalen (1 and 2), S. A. Laurent-Muehleisen (1), E. C. Moran (3), R. H. Becker (1 and 2) ((1) University of California - Davis, (2) LLNL, (3) Wesleyan University)
Comments: 18 pages, 20 figures, Accepted for publication in ApJ

We present results from the analysis of the optical spectra of 47 radio-selected narrow-line Seyfert 1 galaxies (NLS1s). These objects are a subset of the First Bright Quasar Survey (FBQS) and were initially detected at 20 cm (flux density limit ~1 mJy) in the VLA FIRST Survey. We run Spearman rank correlation tests on several sets of parameters and conclude that, except for their radio properties, radio-selected NLS1 galaxies do not exhibit significant differences from traditional NLS1 galaxies. Our results are also in agreement with previous studies suggesting that NLS1 galaxies have small black hole masses that are accreting very close to the Eddington rate. We have found 16 new radio-loud NLS1 galaxies, which increases the number of known radio-loud NLS1 galaxies by a factor of ~5.

 

astro-ph/0601163 [abs, ps, pdf, other] :

Title: Dark Energy Constraints from Baryon Acoustic Oscillations
Authors: Yun Wang
Comments: 16 pages including 4 figures and 1 table. Submitted

Baryon acoustic oscillations (BAO) in the galaxy power spectrum allows us to extract the scale of the comoving sound horizon at recombination, a cosmological standard ruler accurately determined by the cosmic microwave background anisotropy data. We examine various issues important in the use of BAO to probe dark energy. We find that assuming a flat universe, and priors on $\Omega_m$, $\Omega_m h^2$, and $\Omega_b h^2$ as expected from the Planck mission, the constraints on dark energy parameters scale much less steeply with survey area than (area)$^{-1/2}$ for a given redshift range. Dark energy constraints from BAO are very sensitive to the assumed linear scale of matter clustering and the redshift accuracy of the survey. For a BAO survey with $0.5\leq z \leq 2$, $\sigma(R)=0.4$ (corresponding to $k_{max}(z=0)=0.086 h $Mpc$^{-1}$), and $\sigma_z/(1+z)=0.001$, $({\sigma}_{w_0},{\sigma}_{w'})=$(0.115, 0.183) and (0.069, 0.104) for survey areas of 1000 (deg)$^2$ and 10000 (deg)$^2$ respectively. We find that it is critical to minimize the bias in the scale estimates in order to derive reliable dark energy constraints. For a 1000 (10000) square degree BAO survey, a 1$\sigma$ bias in $\ln H(z)$ leads to a 2$\sigma$ (3$\sigma$) bias in $w'$. The bias in $w'$ due to the same scale bias from $\ln D_A(z)$ is slightly smaller and opposite in sign. The results from this paper will be useful in assessing different proposed BAO surveys and guiding the design of optimal dark energy detection strategies.

 

astro-ph/0601164 [abs, ps, pdf, other] :

Title: Hydrodynamic simulations of He-shell flash convection
Authors: Falk Herwig (1), Bernd Freytag (1,2,3), Robert M. Hueckstaedt (1), Francis X. Timmes (1) ((1)Los Alamos National Laboratory, USA (2) Uppsala University, Sweden (3) Michigan State University, USA)
Comments: To appear in ApJ. Submitted Oct 21, 2005. Accepted Dec 29, 2005 . 31 pages, 24 figures. 15 figures in gif format that will not compile in latex. Complete pdf version available for download at this http URL

We present the first hydrodynamic, multi-dimensional simulations of He-shell flash convection. Specifically, we investigate the properties of shell convection at a time immediately before the He- luminosity peak during the 15th thermal pulse of a stellar evolution track with initially two solar masses and metallicity Z=0.01. This choice is a representative example of a low-mass asymptotic giant branch thermal pulse. We construct the initial vertical stratification with a set of polytropes to resemble the stellar evolution structure. Convection is driven by a constant volume heating in a thin layer at the bottom of the unstable layer. We calculate a grid of 2D simulations with different resolutions and heating rates. Our set of simulations includes one low-resolution 3D run. The computational domain includes 11.4 pressure scale heights. He-shell flash convection is dominated by large convective cells that are centered in the lower half of the convection zone. Convective rolls have an almost circular appearance because focusing mechanisms exist in the form of the density stratification for downdrafts and the heating of localized eddies that generate upflows. Nevertheless, downdrafts appear to be somewhat more focused. The He-shell flash convection generates a rich spectrum of gravity waves in both stable layers above and beneath the convective shell. The magnitude of the convective velocities from our 1D mixing-length theory model and the rms-averaged vertical velocities from the hydrodynamic model are consistent within a factor of a few. However, the velocity profile in the hydrodynamic simulation is more asymmetric, and decays exponentially inside the convection zone. [abbreviated]

 

astro-ph/0601165 [abs, ps, pdf, other] :

Title: The near-infrared luminosity function of cluster galaxies beyond redshift one
Authors: V. Strazzullo (Univ. Napoli Federico II, ESO), P. Rosati (ESO), S. A. Stanford (UC Davis, LNLL), C. Lidman (ESO), M. Nonino (INAF - Oss. Astr. Trieste), R. Demarco (JHU), P. E. Eisenhardt (JPL), S. Ettori (INAF - Oss. Astr. Bologna), V. Mainieri (MPE), S. Toft (Yale)
Comments: 16 pages, 10 figures, to appear in A&A

We determined the K band luminosity function (LF) of cluster galaxies at redshift z~1.2, using near-infrared images of three X-ray luminous clusters at z=1.11,1.24,1.27. The composite LF was derived down to M*+4, by means of statistical background subtraction, and is well described by a Schechter function with K*=20.5 AB mag and alpha=-1.
From the K band composite LF we derived the stellar mass function of cluster galaxies. Using available X-ray mass profiles we determined the M/L ratios of these three clusters, which tend to be lower than those measured in the local universe.
With these data, no significant difference can be seen between the shapes of the cluster galaxies LF and the LF of field galaxies at similar redshift.
We also found no significant evolution out to z ~1.2 in the bright (<M*+4) part of the cluster galaxies LF probed in this study, apart from a brightening of ~1.3 mag of the characteristic magnitude of the high redshift LF. We confirm, and extend to higher redshift, the result from previous work that the redshift evolution of the characteristic magnitude M* is consistent with passive evolution of a stellar population formed at z>2.
The results obtained in this work support and extend previous findings that most of the stars in bright galaxies were formed at high redshift, and that K-bright (M>10^11 Msun) galaxies were already in place at z ~ 1.2, at least in the central regions of X-ray luminous clusters. Together with recent results on the field galaxies stellar mass function, this implies that most of the stellar mass is already assembled in massive galaxies by z ~ 1, both in low and high density environments.

 

astro-ph/0601166 [abs, ps, pdf, other] :

Title: Encounter-Triggered Disc Mass Loss in the ONC
Authors: C. Olczak, S.Pfalzner, R.Spurzem
Comments: 32 pages, 12 figures, 3 tables. accepted by ApJ

The relevance of encounters on the destruction of protoplanetary discs in the Orion Nebula Cluster (ONC) is investigated by combining two different types of numerical simulation. First, star-cluster simulations are performed to model the stellar dynamics of the ONC, the results of which are used to investigate the frequency of encounters, the mass ratio and separation of the stars involved, and the eccentricity of the encounter orbits. The results show that interactions that could influence the star-surrounding disc are more frequent than previously assumed in the core of the ONC, the so-called Trapezium cluster. Second, a parameter study of star-disc encounters is performed to determine the upper limits of the mass loss of the discs in encounters. For simulation times of $\sim$ 1-2 Myr (the likely age of the ONC) the results show that gravitational interaction might account for a significant disc mass loss in dense clusters. Disc destruction is dominated by encounters with high-mass stars, especially in the Trapezium cluster, where the fraction of discs destroyed due to stellar encounters can reach 10-15%. These estimates are in accord with observations of (Lada et al. 2000) who determined a stellar disc fraction of 80-85%. Thus, it is shown that in the ONC - a typical star-forming region - stellar encounters do have a significant effect on the mass of protoplanetary discs and thus affect the formation of planetary systems.

 

astro-ph/0601167 [abs, ps, pdf, other] :

Title: The Butcher--Oemler effect at z~0.35: a change in perspective
Authors: S. Andreon, H. Quintana, M. Tajer, G. Galaz, J. Surdej
Comments: MNRAS, in press

The present paper focuses on the much debated Butcher-Oemler effect: the increase with redshift of the fraction of blue galaxies in clusters. Considering a representative cluster sample made of seven group/clusters at z~0.35, we have measured the blue fraction from the cluster core to the cluster outskirts and the field mainly using wide field CTIO images. This sample represents a random selection of a volume complete x-ray selected cluster sample, selected so that there is no physical connection with the studied quantity (blue fraction), to minimize observational biases. In order to statistically assess the significance of the Butcher-Oemler effect, we introduce the tools of Bayesian inference. Furthermore, we modified the blue fraction definition in order to take into account the reduced age of the universe at higher redshifts, because we should no longer attempt to reject an unphysical universe in which the age of the Universe does depend on redshift, whereas the age of its content does not. We measured the blue fraction from the cluster center to the field and we find that the cluster affects the properties of the galaxies up to two virial radii at z~0.35. Data suggest that during the last 3 Gyrs no evolution of the blue fraction, from the cluster core to the field value, is seen beyond the one needed to account for the varying age with redshift of the Universe and of its content. The agreement of the radial profiles of the blue fraction at z=0 and z~0.35 implies that the pattern infall did not change over the last 3 Gyr, or, at least, its variation has no observational effect on the studied quantity.

 

astro-ph/0601168 [abs, ps, pdf, other] :

Title: The Cosmological Parameters 2005
Authors: Ofer Lahav, Andrew R Liddle
Comments: 26 pages TeX file. Article for The Review of Particle Physics 2006 (aka the Particle Data Book), published version at this http URL . This article supersedes astro-ph/0406681
Journal-ref: S. Eidelman et al., Phys. Lett. B 592, 1 (2004) and 2005 partial update for the 2006 edition available at the PDG WWW pages at http://pdg.lbl.gov/

This is a review article for The Review of Particle Physics 2006 (aka the Particle Data Book). It forms a compact review of knowledge of the cosmological parameters as at the end of 2005. Topics included are Parametrizing the Universe; Extensions to the standard model; Probes; Bringing observations together; Outlook for the future.

 

astro-ph/0601169 [abs, ps, pdf, other] :

Title: Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry
Authors: P. Deroo, H. Van Winckel, M. Min, L.B.F.M. Waters, T. Verhoelst, W. Jaffe, S. Morel, F. Paresce, A. Richichi, P. Stee, M. Wittowski
Comments: 12 pages, 10 figures, accepted for publication in A & A

We present the first mid-IR long baseline interferometric observations of the circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD 52961, were observed using the VLTI/MIDI instrument during Science Demonstration Time. Both objects are known binaries for which a stable circumbinary disc is proposed to explain the SED characteristics. This is corroborated by our N-band spectrum showing a crystallinity fraction of more than 50 % for both objects, pointing to a stable environment where dust processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved in the interferometric observations providing an upper limit of 11 mas (or 18 AU at the distance of this object) on the diameter of the dust emission. This confirms the very compact nature of its circumstellar environment. The dust emission around HD 52961 originates from a very small but resolved region, estimated to be ~ 35 mas at 8 micron and ~ 55 mas at 13 micron. These results confirm the disc interpretation of the SED of both stars. In HD 52961, the dust is not homogeneous in its chemical composition: the crystallinity is clearly concentrated in the hotter inner region. Whether this is a result of the formation process of the disc, or due to annealing during the long storage time in the disc is not clear.

 

astro-ph/0601170 [abs, ps, pdf, other] :

Title: A deep wide-field optical survey in the young open cluster Collinder 359
Authors: N. Lodieu (1 and 2), J. Bouvier (3), D. J. James (4), W.-J. de Wit (3), F. Palla (5), M. J. McCaughrean (2 and 6), J.-C. Cuillandre (7) ((1) University of Leicester, UK, (2) Astrophysikalisches Institut Potsdam, Germany, (3) Laboratoire d'Astrophysique, Observatoire de Grenoble, France, (4) Physics and Astronomy Department, Vanderbilt University, USA, (5) Osservatorio Astrofisico di Arcetri, Florence, Italy (6) University of Exeter, UK, (7) Canada-France-Hawaii Telescope, Hawaii, USA)
Comments: 13 pages, 9 figures, 4 tables, Table 4 only available in electronic format, accepted for publication in A&A on 30 Dec 2005

We present the first deep, optical, wide-field imaging survey of the young open cluster Collinder 359, complemented by near-infrared follow-up observations. This study is part of a large programme aimed at examining the dependence of the mass function on environment and time. We have surveyed 1.6 square degrees in the cluster, in the I and z filters, with the CFH12K camera on the Canada-France-Hawaii 3.6-m telescope down to completeness and detection limits in both filters of 22.0 and 24.0 mag, respectively. Based on their location in the optical (I-z,I) colour-magnitude diagram, we have extracted new cluster member candidates in Collinder 359 spanning 1.3-0.03 Msun assuming an age of 60 Myr and a distance of 450 pc for the cluster. We have used the 2MASS database as well as our own near-infrared photometry to examine the membership status of the optically-selected cluster candidates. Comparison of the location of the most massive members in Collinder 359 in a (B-V,V) diagram with theoretical isochrones suggests that Collinder 359 is older than Alpha Per but younger than the Pleiades. We discuss the possible relationship between Collinder 359 and IC 4665 as both clusters harbour similar parameters, including proper motion, distance, and age.

 

astro-ph/0601171 [abs, ps, pdf, other] :

Title: Is space really expanding? A counterexample
Authors: Michal Chodorowski
Comments: 10 pages, no figures; draft version, all comments welcome

In all Friedman models, the cosmological redshift is widely interpreted as a consequence of the general-relativistic phenomenon of EXPANSION OF SPACE. Other commonly believed consequences of this phenomenon are superluminal recession velocities of distant galaxies and the distance to the particle horizon greater than c*t (where t is the age of the Universe), in apparent conflict with special relativity. Here, we study a particular Friedman model: empty universe. This model exhibits both cosmological redshift, superluminal velocities and infinite distance to the horizon. However, we show that the cosmological redshift is there simply a relativistic Doppler shift. Moreover, apparently superluminal velocities and `acausal' distance to the horizon are in fact a direct consequence of special-relativistic phenomenon of time dilation, as well as of the adopted definition of distance in cosmology. There is no conflict with special relativity, whatsoever. In particular, INERTIAL recession velocities are subluminal. Since in the real Universe, sufficiently distant galaxies recede with relativistic velocities, these special-relativistic effects must be at least partly responsible for the cosmological redshift and the aforementioned `superluminalities', commonly attributed to the expansion of space. Let us finish with a question resembling a Buddhism-Zen `koan': in an empty universe, what is expanding?

 

astro-ph/0601172 [abs, ps, pdf, other] :

Title: The role of kink instability in Poynting-flux dominated jets
Authors: Dimitrios Giannios, Henk C. Spruit (Max Planck Institute for Astrophysics)
Comments: 13 pages, 7 figures, accepted for publication in A&A

The role of kink instability in magnetically driven jets is explored through numerical one-dimensional steady relativistic MHD calculations. The instability is shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets. In the case of AGN jets, the flow becomes kinetic flux dominated at distances larger than ~1000 r_g because of the rapid dissipation of Poynting flux. When applied to GRB outflows, the model predicts more gradual Poynting dissipation and moderately magnetized flow at distances of ~10^{16} cm where the deceleration of the ejecta due to its interaction with the external medium is expected. The energy released by the instability can power the compact ``blazar zone'' emission and the prompt emission of GRB outflows with high radiative efficiencies.

 

astro-ph/0601173 [abs, ps, pdf, other] :

Title: X-ray flare in XRF 050406: evidence for prolonged engine activity
Authors: P. Romano, A. Moretti, P.L. Banat (1), D.N. Burrows (2), S. Campana, G. Chincarini, S. Covino (1), D. Malesani (3), G. Tagliaferri (1), S. Kobayashi (2,4), B. Zhang (5), A.D. Falcone (2), L. Angelini (6,7), S. Barthelmy (6), A.P. Beardmore (8), M. Capalbi (9), G. Cusumano (10), P. Giommi (11), M.R. Goad, O. Godet (8), D. Grupe (2), J.E. Hill (6,12), J.A. Kennea (2), V. La Parola, V. Mangano (10), P. Meszaros, D.C. Morris, J. A. Nousek (2), P.T. O'Brien, J.P. Osborne (8), A. Parsons (6), M. Perri (9), C. Pagani (2,1), K.L. Page, A.A. Wells (8), N. Gehrels (6)
Comments: 11 pages, 3 figures, accepted for publication in A&A

We present observations of XRF 050406, the first burst detected by Swift showing a flare in its X-ray light curve. During this flare, which peaks at t_peak ~210s after the BAT trigger, a flux variation of (delta F)/F~6 in a very short time (delta t)/t_peak<<1 was observed. Its measured fluence in the 0.2-10 keV band was ~1.4x10^-8 erg cm^-2, which corresponds to 1-15% of the prompt fluence. We present indications of spectral variations during the flare. We argue that the producing mechanism is late internal shocks, which implies that the central engine is still active at 210s, though with a reduced power with respect to the prompt emission. The X-ray light curve flattens to a very shallow slope with decay index of ~0.5 after ~4400s, which also supports continued central engine activity at late times. This burst is classified as an X-ray flash, with a relatively low fluence (~10^-7 erg cm^-2 in the 15-350 keV band, E_iso~10^51 erg), a soft spectrum (photon index 2.65), no significant flux above ~50 keV and a peak energy E_p<15 keV. XRF 050406 is one of the first examples of a well-studied X-ray light curve of an XRF. We show that the main afterglow characteristics are qualitatively similar to those of normal GRBs. In particular, X-ray flares superimposed on a power-law light curve have now been seen in both XRFs and GRBs. This indicates that a similar mechanism may be at work for both kinds of events.

 

astro-ph/0601174 [abs, ps, pdf, other] :

Title: Interstellar Extinction Law in the J, H, and Ks Bands toward the Galactic Center
Authors: Shogo Nishiyama, Tetsuya Nagata, Nobuhiko Kusakabe, Noriyuki Matsunaga, Takahiro Naoi, Daisuke Kato, Chie Nagashima, Koji Sugitani, Motohide Tamura, Toshihiko Tanabe, Shuji Sato
Comments: 18 pages, 9 figures, Accepted for publication in the ApJ

We have determined the ratios of total to selective extinction in the near-infrared bands (J, H, Ks) toward the Galactic center from the observations of the region |l| < 2.0deg and 0.5deg < |b| < 1.0deg with the IRSF telescope and the SIRIUS camera. Using the positions of red clump stars in color-magnitude diagrams as a tracer of the extinction and reddening, we determine the average of the ratios of total to selective extinction to be A(Ks)/E(H-Ks) = 1.44+-0.01, A(Ks)/E(J-Ks) = 0.494+-0.006, and A(H)/E(J-H) = 1.42+-0.02, which are significantly smaller than those obtained in previous studies. From these ratios, we estimate that A(J) : A(H) : A(Ks) = 1 : 0.573+-0.009 : 0.331+-0.004 and E(J-H)/E(H-Ks) = 1.72+-0.04, and we find that the power law A(lambda) \propto lambda^{-1.99+-0.02} is a good approximation over these wavelengths. Moreover, we find a small variation in A(Ks)/E(H-Ks) across our survey. This suggests that the infrared extinction law changes from one line of sight to another, and the so-called ``universality'' does not necessarily hold in the infrared wavelengths.

 

astro-ph/0601175 [abs, ps, pdf, other] :

Title: The Chandra view of the 3C/FRI sample of low luminosity radio-galaxies
Authors: B. Balmaverde (1), A. Capetti (2), P. Grandi (3) ((1) Universita' di Torino, (2) Osservatorio Astronomico di Torino, (3) INAF di Bologna)
Comments: Accepted in A&A

We present results from Chandra observations of the 3C/FRI sample of low luminosity radio-galaxies. We detected a power-law nuclear component in 12 objects out of the 18 with available data. In 4 galaxies we detected nuclear X-ray absorption at a level of about N_H= (0.2-6)e22 cm-2. X-ray absorbed sources are associated with the presence of highly inclined dusty disks (or dust filaments projected onto the nuclei) seen in the HST images. This suggests the existence of a flattened X-ray absorber, but of much lower optical depth than in classical obscuring tori. We thus have an un-obstructed view toward most FR~I nuclei while absorption plays only a marginal role in the remaining objects. Three pieces of evidence support an interpretation for a jet origin for the X-ray cores: i) the presence of strong correlations between the nuclear luminosities in the radio, optical and X-ray bands, extending over 4 orders of magnitude and with a much smaller dispersion (about 0.3 dex) when compared to similar trends found for other classes of AGNs, pointing to a common origin for the emission in the three bands; ii) the close similarity of the broad-band spectral indices with the sub-class of BL Lac objects sharing the same range of extended radio-luminosity, in accord with the FRI/BL Lacs unified model; iii) the presence of a common luminosity evolution of spectral indices in both FRI and BL Lacs. The low luminosities of the X-ray nuclei, regardless of their origin, strengthens the interpretation of low efficiency accretion in low luminosity radio-galaxies.

 

astro-ph/0601176 [abs, ps, pdf, other] :

Title: A radial velocity survey of low Galactic latitude structures: III. The Monoceros Ring in front of the Carina and Andromeda galaxies
Authors: N. F. Martin, M. J. Irwin, R. A. Ibata, B. C. Conn, G. F. Lewis, M. Bellazzini, S. Chapman, N. Tanvir
Comments: 5 figures, accepted for publication in MNRAS

As part of our radial velocity survey of low Galactic latitude structures that surround the Galactic disc, we report the detection of the so called Monoceros Ring in the foreground of the Carina dwarf galaxy at Galactic coordinates (l,b)=(260,-22) based on VLT/FLAMES observations of the dwarf galaxy. At this location, 20 degrees in longitude greater than previous detections, the Ring has a mean radial velocity of 145+/-5 km/s and a velocity dispersion of only 17+/-5 km/s. Based on Keck/DEIMOS observations, we also determine that the Ring has a mean radial velocity of -75+/-4 km/s in the foreground of the Andromeda galaxy at (l,b)\sim(122,-22), along with a velocity dispersion of 26+/-3 km/s. These two kinematic detections are both highly compatible with known characteristics of the structure and, along with previous detections provide radial velocity values of the Ring over the 120<l<260 range. This should add strong constraints on numerical models of the accretion of the dwarf galaxy that is believed to be the progenitor of the Ring.

 

astro-ph/0601177 [abs, ps, pdf, other] :

Title: Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
Authors: I. Pillitteri (1,2), G. Micela (2), F. Damiani (2), S. Sciortino (2) ((1) DSFA (Universita` degli Studi di Palermo), (2) INAF Osservatorio Astronomico di Palermo)
Comments: 37 pages, 13 figures, to be published on A&A

We report on a deep X-ray survey of the young (~ 140 Myr), rich open cluster NGC 2516 obtained with the EPIC camera on board the XMM-Newton satellite. By combining data from six observations, a high sensitivity, greater than a factor 5 with respect to recent Chandra observations, has been achieved. We detected 431 X-ray sources and 234 of them have as optical counterparts cluster stars spanning the entire NGC 2516 Main Sequence. The X-ray luminosities of cluster stars span the range log Lx (erg/s) = 28.4 - 30.8. The representative temperatures span the 0.3 - 0.6 keV (3.5 - 8 MK) range for the cool component and 1.0 - 2.0 keV (12 -- 23 MK) for the hot one; similar values are found in other young open clusters like the Pleiades, IC 2391, and Blanco 1. While no significant differences are found in X-ray spectra, NGC 2516 solar type stars are definitely less luminous in X-rays than the nearly coeval Pleiades. The comparison with a previous ROSAT survey evidence the lack of variability amplitudes larger than a factor 2 in solar type stars in a ~11 yr time scale of the cluster and thus activity cycles like in the Sun are probably absent or different by period and amplitude in young stars.

 

astro-ph/0601178 [abs, ps, pdf, other] :

Title: New Planetary Nebulae in the Galactic Bulge region with l>0deg - II
Authors: P. Boumis (1), S. Akras (1,2), E. M. Xilouris (1), F. Mavromatakis (3), E. Kapakos (2), J. Papamastorakis (2,4), C. D. Goudis (1,5) ((1) Institute of Astronomy & Astrophysics, National Observatory of Athens, Greece, (2) University of Crete, Heraklion, Crete, Greece, (3) Technological Education Institute of Crete, General Department of Applied Science, Heraklion, Crete, Greece, (4) Foundation for Research and Technology-Hellas, Heraklion, Crete, Greece, (5) Astronomical Laboratory, University of Patras, Rio-Patras, Greece)
Comments: 14 pages, 5 figures, accepted for publication in MNRAS

The presentation of new results from an [O III] 5007 A survey in a search for planetary nebulae (PNe) in the galactic bulge is continued. A total of 60 objects, including 19 new PNe, have been detected in the remaining 34 per cent of the survey area, while 41 objects are already known. Deep Halpha+[N II] CCD images as well as low resolution spectra have been acquired for these objects. Their spectral signatures suggest that the detected emission originates from photoionized nebulae. In addition, absolute line fluxes have been measured and the electron densities are given. Accurate optical positions and optical diameters are also determined.

 

astro-ph/0601179 [abs, ps, pdf, other] :

Title: The Luminosity, Colour and Morphology dependence of galaxy filaments in the Sloan Digital Sky Survey Data Release Four
Authors: Biswajit Pandey, Somnath Bharadwaj
Comments: 12 Pages, 8 figures, Submitted to MNRAS

We have tested for luminosity, colour and morphology dependence of the degree of filamentarity in seven independent nearly two dimensional strips from the Sloan Digital Sky Survey Data Release Four (SDSS DR4). The analysis is carried out at various levels of coarse graining allowing us to address different length- scales. We find that the brighter galaxies have a less filamentary distribution than the fainter ones at all levels of coarse graining. The distribution of red galaxies and ellipticals shows a higher degree of filamentarity compared to blue galaxies and spirals respectively at low levels of coarse graining. The behaviour is reversed at higher levels of coarse graining. We propose a picture where the ellipticals are densely distributed in the vicinity of the nodes where the filaments intersect while the spirals are sparsely distributed along the entire extent of the filaments. Our observations indicate that the regions with an excess of elliptical galaxies are larger than galaxy clusters, protruding into the filaments.

 

astro-ph/0601180 [abs, ps, pdf, other] :

Title: The Ultra Luminous X-ray sources in the High Velocity System of NGC 1275
Authors: O. Gonzalez-Martin, A. C. Fabian, J. S. Sanders
Comments: 7 pages, 4 figures, accepted by MNRAS

We report the results of a study of X-ray point sources coincident with the High Velocity System (HVS) projected in front of NGC 1275. A very deep X-ray image of the core of the Perseus cluster made with the Chandra Observatory has been used. We find a population of Ultra-Luminous X-ray sources (ULX; 7 sources with LX [0.5-7 keV] > 7x10^39 erg/s). As with the ULX populations in the Antennae and Cartwheel galaxies, those in the HVS are associated with a region of very active star formation. Several sources have possible optical counterparts found on HST images, although the X-ray brightest one does not. Absorbed power-law models fit the X-ray spectra, with most having a photon index between 2 and 3.

 

astro-ph/0601181 [abs, ps, pdf, other] :

Title: Optical and Infrared Non-detection of the z=10 Galaxy Behind Abell 1835
Authors: Graham P. Smith, David J. Sand, Eiichi Egami, Daniel Stern, Peter R. Eisenhardt
Comments: Published in ApJ, 636, 575-581

Gravitational lensing by massive galaxy clusters is a powerful tool for the discovery and study of high redshift galaxies, including those at z>=6 likely responsible for cosmic re-ionization. Pello et al. recently used this technique to discover a candidate gravitationally magnified galaxy at z=10 behind the massive cluster lens Abell 1835 (z=0.25). We present new Keck (LRIS) and Spitzer Space Telescope (IRAC) observations of the z=10 candidate (hereafter #1916) together with a re-analysis of archival optical and near-infrared imaging from the Hubble Space Telescope and VLT respectively. Our analysis therefore extends from the atmospheric cut-off at lambda_obs=0.35um out to lambda_obs=5um with Spitzer/IRAC. The z=10 galaxy is not detected in any of these data, including an independent reduction of Pello et al.'s discovery H- and K-band imaging. We conclude that there is no statistically reliable evidence for the existence of #1916. We also assess the implications of our results for ground-based near-infrared searches for gravitationally magnified galaxies at z>~7. The broad conclusion is that such experiments remain feasible, assuming that space-based optical and mid-infrared imaging are available to break the degeneracy with low redshift interlopers (e.g. z~2-3) when fitting spectral templates to the photometric data.

 

astro-ph/0601182 [abs, ps, pdf, other] :

Title: Swift-UVOT Observations of the X-Ray Flash 050406
Authors: P. Schady, K. O. Mason, J. P. Osborne, M. J. Page, P. W. A. Roming, M. Still, B. Zhang, A. J. Blustin, P. Boyd, A. Cucchiara, N. Gehrels, C. Gronwall, M. De Pasquale, S. T. Holland, F. E. Marshall, K. E. McGowan, J. A. Nousek
Comments: 14 pages, 4 figures, accepted for publication in ApJ

We present Swift-UVOT data on the optical afterglow of the X-ray flash of 2005 April 6 (XRF 050406) from 88s to \sim 10^5s after the initial prompt gamma-ray emission. Our observations in the V, B and U bands are the earliest that have been taken of an XRF optical counterpart. Combining the early -time optical temporal and spectral properties with \gamma- and simultaneous X-ray data taken with the BAT and XRT telescopes on-board Swift, we are able to constrain possible origins of the XRF. The prompt emission had a FRED profile (fast-rise, exponential decay) with a duration of T_90 = 5.7\pm 0.2s, putting it at the short end of the long-burst duration distribution. The absence of photoelectric absorption red-ward of 4000 \AA in the UV/optical spectrum provides a firm upper limit of z\leq 3.1 on the redshift, thus excluding a high redshift as the sole reason for the soft spectrum. The optical light curve is consistent with a power-law decay with slope alpha = -0.75\pm 0.26 (F_{\nu}\propto t^{\alpha}), and a maximum occurring in the first 200s after the initial gamma-ray emission. The softness of the prompt emission is well described by an off-axis structured jet model, which is able to account for the early peak flux and shallow decay observed in the optical and X-ray bands.

 

astro-ph/0601183 [abs, ps, pdf, other] :

Title: A 3D radiative transfer framework: I. non-local operator splitting and continuum scattering problems
Authors: Peter H. Hauschildt, E. Baron
Comments: A&A, in press. 14 pages, 19 figures. Full resolution figures available at this ftp URL HTML version (low res figures) at this http URL

We describe a highly flexible framework to solve 3D radiation transfer problems in scattering dominated environments based on a long characteristics piece-wise parabolic formal solution and an operator splitting method. We find that the linear systems are efficiently solved with iterative solvers such as Gauss-Seidel and Jordan techniques. We use a sphere-in-a-box test model to compare the 3D results to 1D solutions in order to assess the accuracy of the method. We have implemented the method for static media, however, it can be used to solve problems in the Eulerian-frame for media with low velocity fields.

 

astro-ph/0601184 [abs, pdf] :

Title: The ECLAIRs micro-satellite mission for gamma-ray burst multi-wavelength observations
Authors: S. Schanne, J.-L. Atteia, D. Barret, S. Basa, M. Boer, F. Casse, B. Cordier, F. Daigne, A. Klotz, O. Limousin, R. Manchanda, P. Mandrou, S. Mereghetti, R. Mochkovitch, S. Paltani, J. Paul, P. Petitjean, R. Pons, G. Ricker, G. Skinner
Comments: 7 pages, 1 figure, proceedings of the conference "New Developments in Photodetection", Beaune (France), June 25005. Submitted to NIM-A (Elsevier Science)

Gamma-ray bursts (GRB), at least those with a duration longer than a few seconds are the most energetic events in the Universe and occur at cosmological distances. The ECLAIRs micro-satellite, to be launched in 2009, will provide multi-wavelength observations of GRB, to study their astrophysics and to use them as cosmological probes. Furthermore in 2009 ECLAIRs is expected to be the only space borne instrument capable of providing a GRB trigger in near real-time with sufficient localization accuracy for GRB follow-up observations with the powerful ground based spectroscopic telescopes available by then. A "Phase A study" of the ECLAIRs project has recently been launched by the French Space Agency CNES, aiming at a detailed mission design and selection for flight in 2006. The ECLAIRs mission is based on a CNES micro-satellite of the "Myriade" family and dedicated ground-based optical telescopes. The satellite payload combines a 2 sr field-of-view coded aperture mask gamma-camera using 6400 CdTe pixels for GRB detection and localization with 10 arcmin precision in the 4 to 50 keV energy band, together with a soft X-ray camera for onboard position refinement to 1 arcmin. The ground-based optical robotic telescopes will detect the GRB prompt/early afterglow emission and localize the event to arcsec accuracy, for spectroscopic follow-up observations.

 

astro-ph/0601185 [abs, ps, pdf, other] :

Title: The true redshift distribution of Pre-SWIFT gamma-ray bursts
Authors: B. Gendre (IASF-Roma/INAF), M. Boer (Observatoire de Haute Provence)
Comments: Proceedings of the "Gamma Ray Bursts in the Swift Era" conference, 6 pages, 5 color figures

SWIFT bursts appear to be more distant than previous bursts. We present the Boer & Gendre relation that link redshift and afterglow luminosities. Taking advantage of the XMM-Newton, Chandra and BeppoSAX catalogs, and using this relation, we have investigated the redshift distribution of GRBs. We find that XMM burst sources with unknown redshift appear to be more distant than those with a known redshift. We propose that this effect may be due to a selection effect of pre-SWIFT optical observations.

 

astro-ph/0601186 [abs, ps, pdf, other] :

Title: Possibility of a photometric detection of "exomoons"
Authors: Gy. M. Szabo, K. Szatmary, Zs. Diveki, A. Simon
Comments: 5 pages, 4 figures, accepted by Astronomy and Astrophysics

We examined which exo-systems contain moons that may be detected in transit. We numerically modeled transit light curves of Earth-like and giant planets that cointain moons with 0.005--0.4 Earth-mass. The orbital parameters were randomly selected, but the entire system fulfilled Hill-stability. We conclude that the timing effect is caused by two scenarios: the motion of the planet and the moon around the barycenter. Which one dominates depends on the parameters of the system. Already planned missions (Kepler, COROT) may be able to detect the moon in transiting extrasolar Earth-Moon-like systems with a 20% probability. From our sample of 500 free-designed systems, 8 could be detected with the photometric accuracy of 0.1 mmag and a 1 minute sampling, and one contains a stony planet. With ten times better accuracy, 51 detections are expected. All such systems orbit far from the central star, with the orbital periods at least 200 and 10 days for the planet and the moon, while they contain K- and M-dwarf stars. Finally we estimate that a few number of real detections can be expected by the end of the COROT and the Kepler missions.

 

astro-ph/0601187 [abs, ps, pdf, other] :

Title: The dynamical state and blue straggler population of the globular cluster NGC 6266 (M62)
Authors: G. Beccari, F.R. Ferraro, A. Possenti, E. Valenti, L. Origlia, R.T. Rood
Comments: 30 pages, 8 figures, accepted for publication in AJ

We have used a proper combination of multiband high-resolution {\it HST-WFPC2} and wide-field ground based observations to image the galactic globular cluster NGC 6266 (M62). The extensive photometric data set allows us to determine the center of gravity and to construct the most extended radial profile ever published for this cluster including, for the first time, detailed star counts in the very inner region. The star density profile is well reproduced by a standard King model with an extended core ($\sim 19''$) and a modest value of the concentration parameter ($c=1.5$), indicating that the cluster has not-yet experienced core collapse.
The millisecond pulsar population (whose members are all in binary systems) and the X-ray emitting population (more than 50 sources within the cluster half mass radius) suggest that NGC 6266 is in a dynamical phase particularly active in generating binaries through dynamical encounters. UV observations of the central region have been used to probe the population of blue straggler stars, whose origin might be also affected by dynamical interactions. The comparison with other globular clusters observed with a similar strategy shows that the blue straggler content in NGC 6266 is relatively low, suggesting that the formation channel that produces binary systems hosting neutron stars or white dwarfs is not effective in significantly increasing the blue straggler population. Moreover, an anticorrelation between millisecond pulsar content and blue straggler specific frequency in globular cluster seems emerging with increasing evidence.

 

astro-ph/0601188 [abs, ps, pdf, other] :

Title: The Complete Spectral Catalog of Bright BATSE Gamma-Ray Bursts
Authors: Yuki Kaneko, Robert D. Preece, Michael S. Briggs, William S. Paciesas, Charles A. Meegan, David L. Band
Comments: To appear in the proceedings of the meeting "Gamma Ray Bursts in the Swift Era"

We present a systematic spectral analysis of 350 bright GRBs observed with BATSE, with high spectral and temporal resolution. Our sample was selected from the complete set of 2704 BATSE GRBs, and included 17 short GRBs. To obtain well-constrained spectral parameters, four different photon models were fitted and the spectral parameters that best represent each spectrum were statistically determined. A thorough analysis was performed on 350 time-integrated and 8459 time-resolved burst spectra. Using the results, we compared time-integrated and time-resolved spectral parameters, and also studied correlations among the parameters and their evolution within each burst. The resulting catalog is the most comprehensive study of spectral properties of GRB prompt emission to date, and provides constraints with exceptional statistics on particle acceleration and emission mechanisms in GRBs.

 

astro-ph/0601189 [abs, ps, pdf, other] :

Title: Searches for diffuse X-ray emission around millisecond pulsars: An X-ray nebula associated with PSR J2124-3358
Authors: C. Y. Hui, W. Becker (MPE)
Comments: Updated version of a paper submitted to A&A Letter on 21 November 2005

We report on diffuse X-ray emission associated with the nearby solitary millisecond pulsar PSR J2124-3358 detected with XMM-Newton and Chandra. The emission extends from the pulsar to the northwest by ~0.5 arcmin. The spectrum of the nebular emission can be modeled with a power-law of photon index $2.2\pm0.3$, in line with the emission originating from accelerated particles in the post shock flow. For PSR J0437-4715, PSR J0030+0451 and PSR J1024-0719, which all have spin parameters comparable to that of PSR J2124-3358, no diffuse emission is detected down to a limiting flux of $2-3\times10^{-15}$ ergs s$^{-1}$ cm$^{-2}$.

 

astro-ph/0601190 [abs, pdf] :

Title: Short Gamma-Ray Bursts with Extended Emission
Authors: Jay P. Norris, Jerry T. Bonnell
Comments: 26 pages, 8 figures, 4 tables. Submitted for publication in The Astrophysical Journal

The recent association of several short gamma-ray bursts (GRBs) with early type galaxies with low star formation rate demonstrates that short bursts arise from a different progenitor mechanism than long bursts. However, since the duration distributions of the two classes overlap, membership is not always easily established. The picture is complicated by the occasional presence of softer, extended emission lasting tens of seconds after the initial spike-like emission. We show that the fundamental defining characteristic of the short burst class is that the initial spike exhibits negligible spectral evolution at energies above ~ 25 keV. This behavior is nearly ubiquitous for the 260 bursts with T90 < 2 s, where the BATSE TTE data completely included the initial spike. The same signature obtains for one HETE-2 and six Swift/BAT short bursts. Analysis of a small sample of "short" BATSE bursts with the most intense extended emission shows that the same lack of evolution on the pulse timescale obtains for the extended emission. The dynamic range in the ratio of peak intensities, spike : extended, is ~ 10^4. For some bursts, the extended emission is only a factor of 2-5 less intense. A high Lorentz factor, ~ 500-1000, might explain the negligible lags observed in short bursts.

 

astro-ph/0601191 [abs, ps, pdf, other] :

Title: Comments on the Cooperstock-Tieu Galaxy Model
Authors: Daniel J. Cross
Comments: 9 pages, 1 figure

The recently proposed Cooperstock-Tieu galaxy model claims to explain the flat rotation curves without dark matter. The purpose of this note is to show that this model is internally inconsistent and thus cannot be considered a valid solution. Moreover, by making the solution consistent the ability to explain the flat rotation curves is lost.