arXiv subjects

Tomasz Bulik

Publications and source records attributed to Tomasz Bulik.

89 records · Page 5Linked to original sources

arXiv2000

Gamma Ray Burst progenitors - a case for helium star mergers

Recently much work in Gamma-Ray Burst (GRB) studies was devoted to revealing the nature of outburst mechanism and to looking for GRB progenitors. Several types of progenitors were proposed for GRBs. Most promising objects are collapsars, compact object binaries, Helium star mergers and recently discussed supernovae. In this paper we consider four proposed binary star progenitors of GRBs: double neutron star (NS-NS), black hole neutron star (BH-NS), black hole white dwarf (BH-WD) mergers and Helium star mergers (He-BH). Helium star mergers are a possible outcome of common envelope evolution of a compact object entering the envelope of a giant with a helium core. Using population synthesis we calculate number of the binary progenitors and show that BH-WD and Helium star mergers dominate population of the proposed binary progenitors. Comparison of distribution of different binary mergers around galaxies they are born in, with localization of GRB afterglows in their host galaxies shows that only Helium mergers may be responsible for GRBs with observed afterglows while it excludes NS-NS, BH-NS and BH-WD systems as GRB progenitors. Assuming that all GRBs come from Helium star mergers and comparing numbers of Helium star mergers with observed BATSE GRB rate let us derive upper limit on GRB collimation to be 4 degrees.

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arXiv2000

Looking for GRB progenitors

Using stellar binary population synthesis code we calculate the production rates and lifetimes of several types of possible GRB progenitors. We consider mergers of double neutron stars, black hole neutron stars, black hole white dwarfs and helium star mergers. We calibrate the results with the measured star formation rate history. We discuss the viability of each GRB model, and alternatively assuming that all bursts are connected with one model we constrain the required collimation of GRBs. We also show the importance of widely used evolutionary parameters on the merger rates of calculated binary populations.

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arXiv2000

Distribution of binary mergers around galaxies

We use a stellar binary population synthesis code to find the lifetimes and velocities of several types of possible GRB progenitors: double neutron stars, black hole neutron stars, black hole white dwarfs, helium star mergers. Assuming that they are born in different types of galaxies we compute their spatial distribution and compare it with the observed locations of GRB afterglows within their hosts. We discuss constraints on the compact object merger model of GRBs imposed by this comparison and find that the observations of afterglows and their host galaxies appear inconsistent with the GRB compact object merger model.

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arXiv1999

Distribution of black hole binaries around galaxies

Compact object mergers are one of the favorite models of GRBs. It has been noted that in opposition to the collapsars, compact object mergers do not necessarily take place in the host galaxies, and may travel outside of them. With the discovery of afterglows and identification of host galaxies one can measure the distribution of GRBs with respect to their host galaxies. This distribution has been calculated using different population synthesis codes, and for different galactic potentials In this paper we compare the distributions of different types of compact object binaries: double neutron star systems (NS-NS), black hole -neutron star systems (BH-NS) and double black holes (BH-BH). We calculate the orbits and distributions of the projected distances on the sky for two extreme cases: a massive galaxy like the Milky Way, and empty space (corresponding to e.g. a globular cluster), and consider a wide range of possible kick velocity distributions. We find that BH-NS are more likely gamma-ray burst counterparts, since they lie close to the host galaxies, contrary to the NS-NS binaries.

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arXiv1999

Constraints on the Spatial Distribution and Luminosity function of GRBs

The distance scale to gamma ray bursts has been a subject of scientific debate for almost thirty years. Up to the discovery of afterglows only indirect methods could be used to constrain the distance scale to this objects. I review some of these results, and show the current limits on the spatial distribution and luminosity function of GRBs. The results obtained with different methods indicate that gamma-ray bursts lie at the typical redshifts $z_{ave}$ between 1 and 2, however there can exist a long tail of the distribution stretching to higher redshifts. The width of the GRB luminosity function (the ratio of the intrinsic brightness of the brightest to the dimmest observed burst) estimated from Beppo SAX bursts with redshifts is almost $10^3$.

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arXiv1999

Distribution of compact object mergers around galaxies

Compact object mergers are one of the currently favored models for the origin of GRBs. The discovery of optical afterglows and identification of the nearest, presumably host, galaxies allows the analysis of the distribution of burst sites with respect to these galaxies. Using a model of stellar binary evolution we synthesize a population of compact binary systems which merge within the Hubble time. We include the kicks in the supernovae explosions and calculate orbits of these binaries in galactic gravitational potentials. We present the resulting distribution of merger sites and discuss the results in the framework of the observed GRB afterglows.

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arXiv1999

The effect of supernova natal kicks on compact object merger rate

Mergers of compact objects may lead to different astrophysical phenomena: they may provide sources of observable gravitational radiation, and also may be connected with gamma-ray bursts. Estimate of the rate with which such mergers take place are based on assumptions about various parameters describing the binary evolution. The distribution of one of these parameters - the kick velocity a neutron star receives at birth will strongly influence the number and orbital parameters of compact objects binaries. We calculate these effect using population synthesis of binary stars and show that the expected compact object merger rate changes by a factor of 30 when the kick velocity varies within its current observational bounds.

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arXiv1998

Photon Splitting in Strongly Magnetized Plasma

The process of photon splitting, becomes allowed in the presence of strong magnetic field. We calculate the influence of magnetized plasma on the photon splitting absorption coefficient. We calculate the refraction coefficients and polarization vectors with the inclusion of the vacuum terms at an arbitrary value of the magnetic field, and then find photon splitting matrix element taking into account the terms that vanish in vacuum, but may be nonzero in the presence of plasma. We find the photon splitting rate in plasma with the density typical for a neutron star star atmosphere and identify a region of the parameter space where the plasma effects are important.

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arXiv1998

Kilohertz QPOs and strange stars

The kilohertz quasi periodic oscillations (QPOs) discovered in several low mass X-ray binaries (LMXBs) by the Rossi X-ray Timing Explorer (XTE) are thought to occur at the orbital frequency in accretion discs whose inner edge corresponds to the innermost (marginally) stable orbit allowed by general relativity. These ideas have been applied to constrain the equation of state (e.o.s.) of the central neutron star. Here we discuss another possibility, that the central object is a strange star, and show how kHz QPOs constrain the e.o.s. of strange matter.

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arXiv1998

Broad-Band Model Spectra of Gamma-Ray Emission from Millisecond Pulsars

We present spectra of pulsed gamma-ray emission expected from millisecond pulsars within the framework of polar-cap models. The spectra are a superposition of three components due to curvature (CR), synchrotron (SR), and Compton upscattering (ICS) processes. The CR component dominates below $100 $GeV and the ICS component exhibits a peak at $\sim 1 $TeV. The CR component should be observable from J0437-4715 with the next generation gamma-ray telescopes, like e.g. GLAST.

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arXiv1998

The Influence of Reprocessing in the Column on the Light Curves of Accretion Powered Neutron Stars

Flow of matter onto strongly magnetized neutron stars in X-ray binaries proceeds through accretion funnels that roughly follow geometry of the magnetic field. X-rays originate near surface of the neutron star, and it may happen that the accretion flow passes through the line of sight as the star rotates. We consider the effects of such accretion flow eclipses on the X-ray light curves of accretion powered pulsars, and present a set of X-ray light-curves measured by BATSE for A0535+262 for which this phenomenon is very likely to take place.

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arXiv1996

Constraints on the Galactic Corona Models of Gamma-Ray Bursts From the 3B Catalogue

We investigate the viability of Galactic corona models of gamma-ray bursts by calculating the spatial distribution expected for a population of high-velocity neutron stars born in the Galactic disk and moving in a gravitational potential that includes the Galactic bulge, disk, and a dark matter halo. We consider models in which the bursts radiate isotropically and in which the radiation is beamed. We place constraints on the models by comparing the resulting brightness and angular distributions with the data in the BATSE 3B catalog. We find that, if the burst sources radiate isotropically, the Galactic corona model can reproduce the BATSE peak flux and angular distributions for neutron star kick velocities > 800 km s$^{-1}$, source turn-on ages > 20 Myrs, and BATSE sampling distances 130 kpc < d_{max} < 350 kpc. If the radiation is beamed, no turn-on age is required and agreement with the BATSE data can be found provided that the width of the beam is less than 20 deg.

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arXiv1996

Spectral Effects of the Vacuum Resonance in Soft Gamma-Ray Repeaters

The association of all three soft gamma-ray repeaters (SGRs) with supernova remnants has established that SGRs are young neutron stars, and has given us a starting point for detailed modeling. One of the most popular classes of models involves strongly magnetised neutron stars, with surface dipole fields B~ 10^{14}-10^{15} Gauss. In such strong magnetic fields, many otherwise negligible processes can play an important role. Here we consider the effects of vacuum polarisation on Compton scattering. Vacuum polarisation introduces a characteristic density-dependent photon frequency at which the normal modes of polarisation become nonorthogonal and the mean free path of photons decreases sharply. Our analytic results and Monte Carlo simulations of photon propagation through a magnetised plasma show that this effect leads, under a wide range of physical conditions, to a broad absorption-like feature in the energy range ~5 keV---40 keV. We discuss this effect in light of the spectra from SGR 1806-20.

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arXiv1995

Compton scattering effects in the spectra of soft gamma-ray repeaters

The association of all three soft gamma-ray repeaters (SGRs) with supernova remnants has made possible estimates of the distance to and luminosity of the sources of SGRs, which have provided a starting point for detailed modeling. One of the most popular classes of models involves strongly magnetized neutron stars, with surface dipole fields $B\sim 10^{14}-10^{15}$ G. In these ``magnetar" models, many otherwise negligible processes can play an important role. Here we consider the spectral effects of strong-field modifications to Compton scattering, in particular those related to the contribution of vacuum polarization to the dielectric tensor. Vacuum polarization introduces a density-dependent photon frequency, called the second vacuum frequency, at which the normal modes of polarization become nonorthogonal and the mean free path of photons decreases sharply. Monte Carlo simulations of photon propagation through a magnetized plasma show that this effect leads, under a wide range of physical conditions, to a broad absorption-like feature in the energy range $\sim$5 keV---40 keV.

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arXiv1995

Polarization Modes in a Strongly Magnetized Hydrogen Gas

We investigate the properties of the normal modes in a strongly magnetized hydrogen gas for conditions expected in atmospheres of isolated neutron stars. We derive and compute the polarizations and absorption coefficients of the normal modes from the polarizabily tensor using both analytical approximations and numerical calculations. We find that the spectral features and anisotropy associated with the bound-bound and bound-free transitions in the magnetized hydrogen are manifested in the polarization characteristics, which affects substantially the spectral and angular dependences of the absorption coefficients of the normal modes and the transfer of radiation in neutron star atmospheres.

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arXiv1995

Gamma-Ray Bursts from High Velocity Neutron Stars

We investigate the viability of the Galactic halo model of $γ$-ray bursts by calculating the spatial distribution of neutron stars born with high velocities in the Galactic disk, and comparing the resulting brightness and angular distribution with the BATSE data. We find that the Galactic halo model can reproduce the BATSE peak flux and angular distribution data for neutron star kick velocities $\simgreat 800$~\kms, source turn-on ages $\simgreat 10$~Myrs, and sampling depths $100{\rm~kpc}\simless d_{max} \simless 400$~kpc.

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arXiv1995

Galactic Models of Gamma-Ray Bursts

We describe observational evidence and theoretical calculations which support the high velocity neutron star model of gamma-ray bursts. We estimate the energetic requirements in this model, and discuss possible energy sources. we also consider radiative processes involved in the bursts.

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