arXiv subjects

Krzysztof Belczynski

Publications and source records attributed to Krzysztof Belczynski.

107 records · Page 6Linked to original sources

arXiv2004

Gravitational Waves from Extragalactic Inspiraling Binaries: Selection Effects and Expected Detection Rates

We examine the selection effects that determine how the population of inspiraling binary compact objects (BCOs) is reflected by those potentially observed with ground-based interferometers like LIGO. We lay the ground-work for the interpretation of future observations in terms of constraints on the real population and, correspondingly, binary star evolution models. To determine the extra-galactic population of inspiraling binaries we combine data on distance and blue luminosity from galaxy catalogs with current models of the galactic BCO mass distribution to simulate the physical distribution of binaries in the nearby universe. We use Monte Carlo methods to determine the fraction of binaries observable by the LIGO detectors from each galaxy as a function of the BCO chirp mass. We examine separately the role of source distance, sky position, time of detection, and binary system chirp mass on detection efficiency and selection effects relevant to the three LIGO detectors. Finally, we discuss the implications of the nearby geography of space on anticipated GW detection and compare our results to previous studies, which have assumed uniform galaxy volume density and fixed chirp mass for binary compact objects. From these considerations, actual BCO inspiral observations or significant upper limits on the coalescence rate anticipated in the near future by ground-based interferometers can be used to improve our knowledge of the galactic binary inspiral rate and to constrain models of radio pulsar characteristics and binary star evolution channels leading to neutron star or black hole binaries.

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arXiv2004

A Comprehensive Study of Young Black Hole Populations

We present theoretical models of black hole (BH) populations in young stellar environments, such as starbursts and young star clusters. Using a population synthesis approach we compute the formation rates and characteristic properties of single and binary BHs for various representative ages and choices of parameters. We find that most of the BHs (typically 80% for an initial 50% binary fraction) are single, but with many originating from primordial binaries (which either merged into a single massive star or were disrupted following a supernova explosion). A smaller but significant fraction (typically 20%) of the BHs remain in binary systems. Main-sequence stars are the most frequent BH companions, but massive BH--BH binaries are the next most numerous group. The most massive BHs found in our simulations reach 80 Msun, and are formed through mergers of massive binary components. If formed in a dense star cluster such a massive stellar BH may become the seed for growth to a more massive (``intermediate-mass'') BH. Although we do not include dynamical interactions, our results provide realistic initial conditions for N-body simulations of dense star clusters (e.g., globular clusters) including primordial BHs.

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arXiv2004

First Stellar Binary Black Holes: Strongest Gravitational Wave Burst Sources

Evolution of first population of massive metal-free binary stars is followed. Due to the low metallicity, the stars are allowed to form with large initial masses and to evolve without significant mass loss. Evolution at zero metallicity, therefore, may lead to the formation of massive remnants. In particular, black holes of intermediate-mass (100-500 Msun) are expected to have formed in early Universe, in contrast to the much lower mass stellar black holes (10 Msun) being formed at present. Following a natural assumption, that some of these Population III stars have formed in binaries, the physical properties of first stellar binary black holes are presented. We find that a significant fraction of such binary black holes coalesces within the Hubble time. We point out that burst of gravitational waves from the final coalescences and the following ringdown of these binary black hole mergers can be observed in the interferometric detectors. We estimate that advanced LIGO detection rate of such mergers is at least several events per year with high signal to noise ratio (>10).

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arXiv2003

Galactic Populations of Ultracompact Binaries

Recent RXTE and Chandra discoveries of low mass X-ray binaries with ultra-short orbital periods have initiated theoretical work on the origins of these peculiar systems. Using the StarTrack population synthesis code the formation and evolution of X-ray ultracompact binaries (UCBs) in the Galactic field are analyzed. The relative number of UCBs with a neutron star or a black hole accretor populating our Galaxy is predicted. Our results demonstrate that standard evolutionary scenarios involving primordial binaries can be sufficient to produce the UCBs in the Galactic field without requiring additional processes associated with the dense stellar environments in the cores of globular clusters. In contrast to previous studies we find that the majority of the immediate progenitors of these systems consist of a hydrogen exhausted donor with an ONeMg white dwarf. The evolution of these systems leads to the accretion induced collapse of the white dwarf to a neutron star, which can play an important role in the formation of a majority of Galactic UCBs. We predict that with an increase in the number of X-ray active UCBs hosting neutron stars by an order of magnitude, a system with a black hole accretor may be found.

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arXiv2002

Formation and Evolution of Microquasar GRS 1915+105

Using the StarTrack population synthesis code we analyze the possible formation channels of the Galactic microquasar GRS 1915+105 harboring the most massive known stellar black hole. We find that we are able to reproduce the evolution and the current state of GRS 1915+105 in a model with lowered wind mass loss rates in massive stars. We conclude that the existence of GRS 1915+105 implies that stellar winds in massive stars are a factor of two weaker, but still within observational bounds, than used in stellar models. We discuss the mass transfer initiated by the massive primary in binaries which are the possible precursors of systems harboring massive black holes. We show that such mass transfer evolution is neither clear case B nor C, but instead is initiated during the core helium burning phase of the massive donor star, the progenitor of the black hole. We also argue that massive black holes do not receive any kicks at their formation and are formed through direct collapse of progenitor star without accompanying supernova explosion.

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arXiv2002

Merger Sites of Double Neutron Stars and their Host Galaxies

Using the StarTrack population synthesis code we analyze the formation channels possibly available to double neutron star binaries and find that they can be richer than previously thought. We identify a group of short lived, tight binaries, which do not live long enough to escape their host galaxies, despite their large center-of-mass velocities. We present our most recent results on all possible evolutionary paths leading to the formation of double neutron stars, calculate their coalescence rates, and also revisit the question of the distribution of merger sites around host galaxies. For a wide variety of binary evolution models and galaxy potentials, we find that most of neutron star mergers take place within galaxies. Our results stem from allowing for radial and common envelope evolution of helium-rich stars (testable in the future with detailed stellar-structure and hydrodynamic calculations) and indicate that double neutron star binaries may not be excluded as Gamma-Ray Burst (GRB) progenitors solely on the basis of their spatial distribution around host galaxies. We also find, in contrast to Bethe & Brown (1998), that in a significant fraction of common envelope (CE) phases neutron stars do not accrete enough material to become black holes, and thus the channels involving CEs are still open for the formation of double neutron stars.

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arXiv2002

A Comprehensive Study of Binary Compact Objects as Gravitational Wave Sources: Evolutionary Channels, Rates, and Physical Properties

A new generation of ground-based interferometric detectors for gravitational waves is currently under construction or has entered the commissioning phase (LIGO, VIRGO, GEO600, TAMA). We study the most promising candidate sources for these detectors: inspiraling double compact objects. We use population synthesis methods to calculate the properties and coalescence rates of: double neutron stars, black hole-neutron star systems and double black holes. We also examine their formation channels. We explicitly account for the evolution of low-mass helium stars and investigate the possibility of common-envelope evolution involving helium stars as well as two evolved stars. As a result we identify a significant number of new formation channels for double neutron stars, in particular, leading to populations with very distinct properties. We discuss the theoretical and observational implications of such populations, but we also note the need for hydrodynamical calculations to settle the question of whether such common-envelope evolution is possible. We also present and discuss the physical properties of compact object binaries and identify a number of robust, qualitative features as well as their origin. Using the calculated coalescence rates we compare our results to earlier studies and derive expected detection rates for LIGO. We find that our most optimistic estimate for the first LIGO detectors reach a couple of events per year and our most pessimistic estimate for advanced LIGO detectors exceed 10 events per year.

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arXiv2002

Population synthesis of compact objects: neutron stars, strange (quark) stars and black holes

We compute and present the distribution in mass of single and binary neutron stars, strange stars, and black holes. The calculations were performed using a stellar population synthesis code. We follow evolution of massive single stars as well as binaries with high mass primaries. The final product of the latter evolution can be either a binary composed of a white dwarf and a compact object (neutron star, black hole, strange star), two compact objects in a binary, or two single stars if the system was disrupted. We find in binaries a population of black holes which are more massive than single black holes which are a product of either binary or single evolution. We also find that if quark stars exist at all, their population can be as large as the population of black holes.

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arXiv2002

Short GRBs and Mergers of Compact Objects: Observational Constraints

GRB data accumulated over the years have shown that the distribution of their time duration is bimodal. While there is some evidence that long bursts are associated with star-forming regions, nothing is known regarding the class of short bursts. Their very short timescales are hard to explain with the collapse of a massive star, but would be naturally produced by the merger of two compact objects, such as two neutron stars (NS), or a neutron star and a black hole (BH). As for the case of long bursts, afterglow obervations for short bursts should help reveal their origin. By using updated population synthesis code calculations, we simulate a cosmological population of merging NS-NS and NS-BH, and compute the distribution of their galactic off-sets, the density distribution of their environment, and, if indeed associated with GRBs, their expected afterglow characteristics.

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arXiv2001

Study of Gamma Ray Burst Binary Progenitors

Recently much work in studying Gamma-Ray Burst has been devoted to revealing the nature of outburst mechanism and studies of GRB afterglows. These issues have also been closely followed by the quest for identifying GRB progenitors. In this paper we consider the proposed binary star progenitors of GRBs: white dwarf neutron star binaries, white dwarf black hole binaries, helium core neutron star mergers, helium core black hole mergers, double neutron stars and neutron star black hole binaries. Using population synthesis methods we calculate merger rates of these binary progenitors and we compare them to the observed BATSE GRB rate. We also calculate the distribution of merger sites around host galaxies and compare them to the observed locations of GRB afterglows with respect to their hosts. We find that the rates of binary GRB progenitors in our standard model are lower than the observed GRB rates if GRBs are highly collimated. However, the uncertainty in the population synthesis results is too large to make this a firm conclusion. Although some observational signatures seem to point to collapsars as progenitors of long GRBs, we find that mergers of WD-NS, He-NS, He-BH, and NS-NS systems also trace the star formation regions of their host galaxies, as it is observed for long GRBs. We also speculate about possible progenitors of short-duration GRBs. For these, the most likely candidates are still mergers of compact objects. We find that the locations NS-NS and NS-BH mergers with respect to their hosts are significantly different. This may allow to distinguish between these two progenitor models, once current and near future missions, such as HETE-II or SWIFT, measure the locations of short GRBs.

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arXiv2000

Population synthesis of neutron stars, strange (quark) stars and black holes

We compute and present the distribution in mass of single and binary neutron stars, strange stars, and black holes. The calculations were performed using a stellar population synthesis code. We follow all phases of single and binary evolution, starting from a ZAMS binary and ending in the creation of one compact object (neutron star, black hole, strange star) and a white dwarf, or two compact objects (single or binary). We assume that neutron stars are formed in the collapse of iron/nickel cores in the mass range M0 < M < M1, quark stars in the range M1 < M < M2, and black holes for core masses M > M2 and find that the population of quark stars can easily be as large as the population of black holes, even if there is only a small mass window for their formation.

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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

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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