arXiv subjects

T. Bulik

Publications and source records attributed to T. Bulik.

At least 235 records · Page 13Linked to original sources

arXiv2003

Constraining Binary Evolution with Gravitational Wave Measurements of Chirp Masses

Using the StarTrack binary population synthesis code we investigate the properties of population of compact object binaries. Taking into account the selection effects we calculate the expected properties of the observed binaries.We analyze possible constraints on the stellar evolution models and find that an observed sample of about one hundred mergers will yield strong constraints on the binary evolution scenarios.

astro-ph↗

arXiv2003

Astrophysical significance of detection of coalescing binaries with gravitational waves

We use the StarTrack stellar population synthesis code to analyze properties of double compact object binaries as sources of gravitational waves. Since the distribution of lifetimes of these objects extends up to the Hubble time we conclude that a proper calculation of the expected rate must include a full cosmological model. We present such model, calculate the expected coalescence rates, and analyze the intrinsic sensitivity of these rates to the model assumptions. We find that the rate alone is a very poor estimator of the underlying stellar evolution model. However we show that the distribution of observed chirp masses is very sensitive to the underlying stellar evolution model, while it is very insensitive to the underlying cosmology, star formation rate history and variation of detector sensitivity.

astro-ph↗

arXiv2003

Constraints on the binary evolution from chirp mass measurements

We estimate the observed distribution of chirp masses of compact object binaries for the gravitational wave detectors. The stellar binary evolution is modeled using the {\em StarTrack} population synthesis code. The distribution of the predicted "observed" chirp masses vary with variation of different parameters describing stellar binary evolution. We estimate the sensitivity of the observed distribution to variation of these parameters and show which of the parameters can be constrained after observing 20, 100, and 500 compact object mergers. As a general feature of all our models we find that the population of observed binaries is dominated by the double black hole mergers.

astro-ph↗

arXiv2002

A Puzzling Paucity of Double Peaked X-ray Pulsars

Accretion powered pulsars exhibit a variety of lightcurves. In this paper we propose to classify the observed lightcurves as single or double pulsed.We analyze the lightcurves of 86 accretion powered pulsars and assign them to these classes. We present three datasets: first in which the classification can be easily done, second for which the classification is more difficult and not certain, and third for which we were unable to classify the pulsar because of lack of published data. We analyze a simple model in which the angles between the magnetic and rotation axis $β$, and between the rotation axis and the line of sight $θ$ are random, and show that it is inconsistent with the data. We also present a model in whichthe angle between the magnetic axis and rotation axis is restricted and compare it with the data. This leads to an upper limit on the angle $β< 40^\circ$. We conclude that there must be a mechanism that leads to alignment of the magnetic and spin axis in X-ray pulsars.

astro-ph↗

arXiv2002

The distribution of mass ratios in compact object binaries

Using the StarTrack population synthesis code we compute the distribution of masses of merging compact object (black hole or neutron star) binaries. The shape of the mass distribution is sensitive to some of the parameters governing the stellar binary evolution. We discuss the possibility of constraining stellar evolution models using mass measurements obtained from the detection of compact object inspiral with the upcoming gravitational-wave observatories.

astro-ph↗

arXiv2002

Evidence for a 2:3 resonance in Sco X-1 kHz QPOs

We find evidence that the two high frequency QPOs in Sco X-1 are, more often than not, approximately in the 2:3 frequency ratio familiar from studies of black hole candidates (e.g., XTE J1550-564, Remillard et al. 2002). This implies that the double kHz QPO phenomenon in neutron stars has its origin in properties of strong-field gravity and has little to do with the rotation of a stellar surface or any magnetic field structure anchored in the star.

astro-ph↗

arXiv2002

High-energy radiation from pulsars: a challenge to polar-cap models

Spectacular results of pulsar observations in X-ray and gamma-ray domains gave a new boost to theoretical models of pulsar magnetospheric activity. A challenging aspect of these efforts is that lightcurves and broadband energy spectra of the brightest HE sources exhibit unexpected richness of features, making each object almost a unique case to be interpreted with a custom-made model. This review offers our subjective account of how the models of high-energy radiation used within the framework of SCLF polar-cap scenarios tackle these challenges. We describe major characteristics of all radiative processes relevant for modeling the observed features. Then we address successes as well as noticeable disadvantages of these models upon their confrontation with the available data.

astro-ph↗

arXiv2001

Lower limits on the maximum orbital frequency around rotating strange stars

Observations of kHz quasi-periodic oscillations (QPOs) in the X-ray fluxes of low-mass X-ray binaries (LMXBs) have been used in attempts to constrain the external metric of the compact members of these binaries, as well as their masses and the equation of state of matter at supranuclear denisties. We compute the maximum orbital frequency of stable circular motion around uniformly rotating strange stars described by the MIT bag model. The calculations are performed for both normal and supramassive constant baryon mass sequences of strange stars rotating at all possible rates. We find the lower limits on the maximum orbital frequency and discuss them for a range of masses and for all rotational frequencies allowed in the model considered. We show that for slowly and moderately rotating strange stars the maximum value of orbital frequency can be a good indicator of the mass of the compact object. However, for rapidly rotating strange stars the same value of orbital frequency in the innermost stable circular orbit is obtained for stars with masses ranging from that of a planetoid to about three solar masses. At sufficiently high rotation rates of the strange star, the rotational period alone constrains the stellar mass to a surprisingly narrow range.

astro-ph↗

arXiv2000

Innermost stable circular orbits around rotating compact quark stars and QPOs

It has been suggested that observations of quasi periodic oscillations (QPOs) in the X-ray fluxes from low mass X-ray binaries can be used to constrain the mass of the compact object and the equation of state of its matter. A specific suggestion that the kHz QPO frequency saturates at the maximum orbital frequency has been widely considered. We examine rotating compact stars described by a new model of strange quark matter (Dey et al. 1998). We calculate the maximum orbital frequencies for both normal and supramassive strange stars described by the Dey model, and present these frequencies for sequences of equilibrium models with constant baryon mass. The maximum orbital frequencies for these compact objects are always higher than the kHz QPO frequencies observed to date.

astro-ph↗

arXiv2000

Low-mass quark stars as Maclaurin spheroids

We show that in the limit of low masses (M<0.1M solar), the orbital and rotational frequencies in exact relativistic numerical models of rotating, axially symmetric, quark stars coincide with those for Maclaurin spheroids. In particular, when the eccentricity of the spheroid exceeds a critical value (e>0.83458318), circular orbits in the equatorial plane are unstable for a range of orbital radii outside the stellar surface. The orbital frequency in the marginally stable orbit, and all other orbits, around Maclaurin spheroids goes to zero in the limit e=1. Maximum orbital frequencies in accreting Galactic X-ray sources may be a poor indication of the source mass.

astro-ph↗

arXiv2000

Model Spectra of Rotation Powered Pulsars in the INTEGRAL Range

The energy range of IBIS is a promising ground for testing mutual relations of distinct components expected in the spectra of high-energy radiation from rotation powered pulsars. According to some polar-cap models two such components - due to curvature and synchrotron emission - may contribute comparable amounts of power between 15 keV and 10 MeV (Rudak & Dyks 1999). Zhang & Harding (2000) argued recently for the inclusion of a third possible component, due to inverse Compton scattering (ICS) of soft thermal photons on secondary $\epm$-pairs. Here we present the results of Monte Carlo calculations of all three spectral components within a polar-cap model which allows for interactions of relativistic particles with the soft photons coming from the pulsar surface. For teragauss pulsars with the surface temperature of a few times 10^5 K the ICS component dominates the spectrum in the energy range below 10 MeV, and thus its presence increases the ratio of X-ray to gamma-ray luminosity (in comparison to the models ignoring the ICS on secondary $\epm$-pairs) to a level observed in the Vela pulsar.

astro-ph↗

arXiv2000

Rotating compact strange stars

We compute numerical models of uniformly rotating strange stars (SS) in general relativity for the recently proposed QCD-based equation of state (EOS) of strange quark matter (Dey et al. 1998). Static models based on this EOS are characterised by a larger surface redshift than strange stars within the MIT bag model. The frequencies of the fastest rotating configurations described by Dey model are much higher than these for neutron stars (NS) and for the simplest SS MIT bag model. We determine a number of physical parameters for such stars and compare them with those obtained for NS. We construct constant baryon mass equilibrium sequences both normal and supramassive. Similarly to the NS a supramassive SS, prior to collapse to a black hole, spins up as it loses angular momentum. We find the upper limits on maximal masses and maximal frequencies of the rotating configurations. We show that the maximal rotating frequency for each of considered evolutionary sequences is never the Keplerian one. A normal and low mass supramassive strange stars gaining angular momentum always slows down just before reaching the Keplerian limit. For a high mass supramassive SS sequence the Keplerian configuration is the one with the lowest rotational frequency in the sequence. The value of $T/W$ for rapidly rotating SS of any mass is significantly higher than those for ordinary NS. For Keplerian configurations it increases as mass decreases. The results are robust for all linear self-bound equations of state.

astro-ph↗

arXiv2000

On the mass of moderately rotating strange stars in the MIT bag model and LMXBs

We compute the maximum mass of moderately rotating strange stars as a function of the strange quark mass, of the QCD coupling constant, $α_c$, and of the bag constant (vacuum energy density), $B$, in the MIT bag model of quark matter with lowest order quark-gluon interactions. For a fixed value of $B$, the maximum stellar mass depends only weakly on $α_c$, and is independent of this coupling in the limit of massless quarks. However, if it is the value of the chemical potential of quark matter at zero pressure which is held constant, for example at the value corresponding to the stability limit of nucleons against conversion to quark matter, the maximum mass of the strange star is higher by up to 25% for $α_c=0.6$, than for non-interacting quarks, and this may be relevant in the discussion of kHz QPO sources. The maximum mass of a non-rotating strange star could be sufficiently high to allow an orbital frequency as low as 1.0 kHz in the marginally stable orbit. However, for all $α_c<0.6$, the stellar mass cannot exceed $2.6M_\odot$ at any rotational period $\ge1.6 $ms.

astro-ph↗

arXiv1999

Spectral features in gamma-rays expected from millisecond pulsars

In the advent of next generation gamma-ray missions, we present general properties of spectral features of high-energy emission above $1\MeV$ expected for a class of millisecond, low magnetic field ($\sim 10^9\G$) pulsars. We extend polar-cap model calculations of Rudak & Dyks (1999) by including inverse Compton scattering events in ambient field of thermal X-ray photons and by allowing for two models of particle acceleration. In the range between $1\MeV$ and a few hundred GeV the main spectral component is due to curvature radiation of primary particles. Synchrotron component due to secondary pairs becomes dominant only below $1\MeV$. The slope of the curvature radiation spectrum in the energy range from $100\MeV$ to $10\GeV$ strongly depends on the model of longitudinal acceleration, whereas below $\sim 100\MeV$ all slopes converge to a unique value of 4/3 (in a $ν{\cal F}_ν$ convention). The thermal soft X-ray photons, which come either from the polar cap or from the surface, are Compton upscattered to a domain of the VHE and form a separate spectral component peaking at $\sim 1\TeV$. We discuss observability of millisecond pulsars by future high energy instruments and present two rankings relevant for GLAST and MAGIC. We point to the pulsar J0437-4715 as a promising candidate for observations.

astro-ph↗

arXiv1999

Modelling blazar radiation spectra Electron injection break and pair content of quasar jets

We study the dependence of nonthermal radiation spectra in OVV quasars on location of the low energy break in the electron/positron injection function. We show that the high energy spectra produced during the outbursts are presumably superposed from two components, one resulting from Comptonization of emission lines, which dominates at MeV-GeV energies, and the other resulting from Comptonization of infrared radiation, which dominates in the X-ray band.

astro-ph↗

arXiv1999

Keplerian frequencies and innermost stable circular orbits of rapidly rotating strange stars

It has been suggested that the frequency in the co-rotating innermost stable circular orbit (ISCO) about a compact stellar remnant can be determined through X-ray observations of low-mass X-ray binaries, and that its value can be used to constrain the equation of state of ultradense matter. Upon constructing numerical models of rapidly rotating strange (quark) stars in general relativity, we find that for stars rotating at the equatorial mass-shedding limit, the ISCO is indeed above the stellar surface, for a wide range of central energy densities at a height equal to 11% of the circumferential stellar radius, which scales inversely with the square root of the energy density, of self-bound quark matter at zero presure. In contrast to static stars, the ISCO frequencies of rapidly rotating strange stars can be as low as 0.9 kHz for a 1.3 solar mass strange star. Hence, the presence of strange stars in low-mass X-ray binaries cannot be excluded on the basis of the currently observed frequencies of kHz QPOs, such as the cut-off frequency of 1066 Hz in 4U 1820-30.

astro-ph↗

arXiv1999

A BeppoSAX study of the pulsating transient X0115+63: the first X-ray spectrum with four cyclotron harmonic features

The recurrent hard pulsating X-ray transient §was observed with \B on March 19, when the source was at a 2--10 keV flux level of $\sim$310~mCrab. We report on the high energy spectrum of the source, concentrating on cyclotron resonant scattering features. The spectrum is strongly pulse phase dependent and absorption features are detected at virtually all phases. In particular, four absorption-like features at 12.74 keV, 24.16 keV, 35.74 keV, and 49.5 keV are observed in the descending edge of the main peak of the pulse profile. The ratios between the centroid energies of the lines with respect to the first are 1:(1.9):(2.8):(3.9). These values are close to the harmonic relation expected from cyclotron resonant scattering in a strong magnetic field, when relativistic effects are taken into account. The equivalent widths of the second, third, and fourth harmonics are found to be larger than that of the first harmonic, confirming the key role of two-photon processes in the spectral formation. These results provide the first evidence for {\it four harmonically spaced lines} in the spectrum of an accreting X-ray pulsar, yielding the clearest confirmation to date of their magnetic origin.

astro-ph↗

arXiv1999

On Beaming Effects in Afterglow Light Curves

The most luminous GRBs can be explained in terms of models involving stellar mass central engines only if the ejecta are beamed. As was pointed out by Rhoads, the dynamics of the blast wave, formed by the beamed ejecta sweeping the external gas, can be significantly modified by the sideways expansion. This is because in this case the surface of the blast wave increases faster than just due to the radial divergence and so the blast wave deceleration rate increases faster. According to analytical estimates, the effect becomes important shortly after the bulk Lorentz factor of the blast wave drops below the inverse of the initial opening angle of the beamed ejecta and is accompanied by a sharp break in the afterglow light curve. However, our numerical studies, which follow the dynamical evolution of the blast wave, the evolution of the electron energy distribution, and take into account the light travel effects related to the lateral size of the source, show that the break of the light curve is weaker and much smoother than the one analytically predicted. A prominent break emerges only for a model without sideways expansion.

astro-ph↗