arXiv subjects

K. Belczynski

Publications and source records attributed to K. Belczynski.

At least 91 records · Page 5Linked to original sources

arXiv2005

Bounds on Expected Black Hole Spins in Inspiraling Binaries

As a first step towards understanding the angular momentum evolution history of black holes in merging black-hole/neutron-star binaries, we perform population synthesis calculations to track the distribution of accretion histories of compact objects in such binaries. We find that there are three distinct processes which can possibly contribute to the black-hole spin magnitude: a birth spin for the black hole, imparted at either (i) the collapse of a massive progenitor star to a black hole or (ii) the accretion-induced collapse of a neutron star to a black hole; and (iii) an accretion spin-up when the already formed black hole [via (i) or (ii)] goes through an accretion episode (through an accretion disk or a common-envelope phase). Our results show that, with regard to accretion-induced spinup in merging BH-NS binaries [method (iii) above], only {\em accretion episodes associated with common-envelope phases and hypercritical accretion rates} occur in the formation history of merging black hole/neutron star binaries. Lacking unambiguous experimental information about BH birth spins [i.e., regarding the results of processes (i) and (ii)], we choose two fiducial values for the BH birth angular momentum parameter a=J/M^2, consistent with observations of (i) NS birth spins (a roughly 0) and (ii) X-ray binaries (a=0.5). Using these two fiducial values and a conservative upper bound on the specific angular momentum of accreted matter, we discuss the expected range of black hole spins in the binaries of interest. We conclude with comments on the significance of these results for ground-based gravitational-wave searches of inspiral signals from black hole binaries.

astro-ph↗

arXiv2005

Constraining population synthesis models via the binary neutron star population

The observed sample of double neutron-star (NS-NS) binaries presents a challenge to population-synthesis models of compact object formation: the parameters entering into these models must be carefully chosen so as to match (i) the observed star formation rate and (ii) the formation rate of NS-NS binaries, which can be estimated from the observed sample and the selection effects related to the discoveries with radio-pulsar surveys. In this paper, we select from an extremely broad family of possible population synthesis models those few (2%) which are consistent with the observed sample of NS-NS binaries. To further sharpen the constraints the observed NS-NS population places upon our understanding of compact-object formation processes, we separate the observed NS-NS population into two channels: (i) merging NS-NS binaries, which will inspiral and merge through the action of gravitational waves within $10 $ Gyr, and (ii) wide NS-NS binaries, consisting of all the rest. With the subset of astrophysically consistent models, we explore the implications for the rates at which double black hole (BH-BH), black hole-neutron star (BH-NS), and NS-NS binaries will merge through the emission of gravitational waves.

astro-ph↗

arXiv2005

Are Supernova Kicks Responsible for X-ray Binary Ejection from Young Clusters?

Recent Chandra observations of interacting and starburst galaxies have led us to investigate the apparent correlation between the positions of young star clusters and Chandra point sources. Assumed to be X-ray binaries (XRBs), these point sources do not seem to coincide with the massive (~1e5 Msun), young (1-50 Myr) stellar clusters that can easily form systems capable of such emission. We use a sophisticated binary evolution and population synthesis code (StarTrack) and a simplified cluster model to track both the X-ray luminosity and position of XRBs as a function of time. These binaries are born within the cluster potential with self-consistent positions and velocities and we show that a large fraction (~70%) can be ejected from the parent due to supernova explosions and associated systemic velocities. For brighter sources and cluster masses below ~1e6 Msun, we find that the average number of bright XRBs per cluster remains near or below unity, consistent with current observations.

astro-ph↗

arXiv2005

The Evolution of Binary Fractions in Globular Clusters

We study the evolution of binary stars in globular clusters using a new Monte Carlo approach combining a population synthesis code (StarTrack), and a simple treatment of dynamical interactions in the dense cluster core using a new tool for computing 3-body and 4-body interactions (Fewbody). We find that the combination of stellar evolution and dynamical interactions (binary-single and binary-binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster like 47 Tuc, assuming an initial binary fraction of 100%, is only about 5-10%. We show that this is in good agreement with recent HST observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modeling of globular clusters, suggesting that ``realistic models'' should incorporate much larger initial binary fractions than has usually been done in the past.

astro-ph↗

arXiv2004

Mapping inspiral rates on population synthesis parameters

Formation rates of compact-object binaries are often derived from population synthesis calculations. However, such calculations depend sensitively on a relatively large number of model input parameters. Given considerable uncertainty in those model parameters, the predicted inspiral rates for double compact objects relevant to gravitational-wave interferometric detectors have been shown to be are uncertain by several orders of magnitude. Typically, inspiral rates are estimated for only a small set of models with a remarkably poor coverage of the highly multi-dimensional parameter space (primarily because of limited computer resources). Here, using as an example seven population-synthesis model parameters, we show that it is possible to derive fits of double-compact-object inspiral rates dependent simultaneously on all seven parameters. We find these fits to be accurate to 50% for binary black holes and to 40% for binary neutron stars. The availability of such fits implies that (i) depending on the problem of interest, it is not necessary to complete large numbers of computationally demanding population synthesis calculations; and (ii) for the first time, the sufficient exploration of the relevant phase space and the assessment of the uncertainties involved is not limited by computational resources and becomes feasible. Finally, we have also produced a histogram of the (a priori likely) binary black hole inpsiral rate, assuming our population synthesis models are equally likely. This histogram, effectively an a priori probability distribution for the BH-BH inspiral rate, suggests that merger rate is conservatively bounded below by 10^(-8)/yr/Milky-Way-galaxy.

astro-ph↗

arXiv2004

Expected masses of merging compact object binaries observed in gravitational waves

We use the well tested StarTrack binary population synthesis code to examine the properties of the population of compact object binaries. We calculate the distribution of masses and mass ratios taking into account weights introduced by observability in gravitational waves during inspiral. We find that in the observability weighted distribution of double neutron star binaries there are two peaks: one for nearly equal mass systems, and one for the systems consisting of a low and a high mass neutron star, $q=0.6-0.7$. The observability weighted distribution of black hole neutron star binaries is concentrated on the systems with the mass ratio $q=0.3-0.5$, while for the double black hole binaries the observability weighted distribution is dominated by the massive, nearly equal mass binaries with $q>0.7$.

astro-ph↗

arXiv2003

On the Maximum Binary Fraction in Globular Cluster Cores

We study the evolution of binary stars in globular clusters using a novel approach combining a state-of-the-art population synthesis code with a simple treatment of dynamical interactions in the dense cluster core. We find that the combination of stellar evolution and dynamical interactions (binary-single and binary-binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster like 47 Tuc, assuming an initial binary fraction of 100%, is only about 5%. We show that this is in good agreement with recent HST observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modeling of globular clusters, suggesting that ``realistic models'' should incorporate much larger initial binary fractions than has usually been done in the past.

astro-ph↗

arXiv2003

X-Ray Binary Populations: The Luminosity Function of NGC1569

Using the population synthesis code StarTrack we construct the first synthetic X-ray binary populations for direct comparison with the X-ray luminosity function (XLF) of NGC 1569 observed with Chandra. Our main goal is to examine whether it is possible to reproduce the XLF shape with our models, given the current knowledge for the star-formation history of this starburst galaxy. We thus produce hybrid models meant to represent the two stellar populations: one old, metal-poor with continuous star-formation for 1.5 Gyr and another recent and metal-rich population. To examine the validity of the models we compare XLFs calculated for varying ages of the populations and varying relative weights for the star-formation rates in the two populations. We find that, for typical binary evolution parameters, it is indeed possible to quite closely match the observed XLF shape. The robust match is achieved for an age of the young population and a ratio of star formation rates in the two populations that are within factors of 1.5 and 2, respectively, of those inferred from HST observations of NGC 1569. In view of this encouraging first step, we discuss the implications of our X-ray binary models and their potential as tools to study binary populations in galaxies.

astro-ph↗

arXiv2003

Compact object mergers as progenitors of short bursts

Compact object mergers are possible progenitors of short bursts. We analyze properties of compact object mergers using the StarTrack population synthesis code, and find that the double neutron star population is dominated by short lived systems, thus they merge within host galaxies, while black hole neutron star binaries merge outside of the host galaxies. We summarize the differences between these two types of mergers.

astro-ph↗

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

The Role of Helium Stars in the Formation of Double Neutron Stars

We have calculated the evolution of 60 model binary systems consisting of helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star companion to investigate the formation of double neutron star systems.Orbital periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe overflow starting from the helium main sequence to after the ignition of carbon burning in the core. We have also examined the evolution into a common envelope phase via secular instability, delayed dynamical instability, and the consequence of matter filling the neutron star's Roche lobe. The survival of some close He-star neutron-star binaries through the last mass transfer episode (either dynamically stable or unstable mass transfer phase) leads to the formation of extremely short-period double neutron star systems (with P<~0.1days). In addition, we find that systems throughout the entire calculated mass range can evolve into a common envelope phase, depending on the orbital period at the onset of mass transfer. The critical orbital period below which common envelope evolution occurs generally increases with M_He. In addition, a common envelope phase may occur during a short time for systems characterized by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun). The existence of a short-period population of double neutron stars increases the predicted detection rate of inspiral events by ground-based gravitational-wave detectors and impacts their merger location in host galaxies and their possible role as gamma-ray burst progenitors. We use a set of population synthesis calculations and investigate the implications of the mass-transfer results for the orbital properties of DNS populations.

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

The spectroscopic orbits and other parameters of the symbiotic binary FN Sgr

We present an analysis of optical and near infrared spectra of the eclipsing symbiotic system FN Sgr. In particular, we have determined for the first time spectroscopic orbits based of the radial velocity curves for both components. A set of blue absorption lines resembling an A-F type star is present in all our spectra. They seem to be associated with the hot component but the best orbital fitting corresponds to a larger eccentricity. We have also studied spectral changes and photometric variations in function of both the hot component activity and the orbital motion.

astro-ph↗

arXiv2002

The spectroscopic orbit and other parameters of AE Arae

We present an analysis of optical and near infrared spectra of the symbiotic system AE Ara, composed of an M5.5 III giant similar to the Galactic Bulge M giants and a hot luminous companion. In particular, we have determined for the first time spectroscopic orbits based of the radial velocity curves for the red and the hot component from the M-giant absorption lines and from the wings of He II 4686 emission profiles, respectively. We have also studied spectral changes and photometric variations in function of both the orbital phase and activity. The resulting physical parameters of the binary components and the nature of the hot component activity are briefly discussed.

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↗

arXiv2001

Binary Population Synthesis: Methods, Normalization, and Surprises

In this paper we present a brief overview of population synthesis methods with a discussion of their main advantages and disadvantages. In the second part, we present some recent results from synthesis models of close binary compact objects with emphasis on the predicted rates, their uncertainties, and the model input parameters the rates are most sensitive to. We also report on a new evolutionary path leading to the formation of close double neutron stars (NS), with the unique characteristic that none of the two NS ever had the chance to be recycled by accretion. Their formation rates turn out to be comparable to or maybe even higher than those of recycled NS-NS binaries (like the ones observed), but their detection probability as binary pulsars is much smaller because of their short lifetimes. We discuss the implications of such a population for gravitational-wave detection of NS-NS inspiral events, and possibly for gamma-ray bursts and their host galaxies.

astro-ph↗

arXiv2001

A New Formation Channel for Double Neutron Stars Without Recycling: Implications for Gravitational Wave Detection

We report on a new evolutionary path leading to the formation of close double neutron stars (NS), with the unique characteristic that none of the two NS ever had the chance to be recycled by accretion. The existence of this channel stems from the evolution of helium-rich stars (cores of massive NS progenitors), which has been neglected in most previous studies of double compact object formation. We find that these non-recycled NS-NS binaries are formed from bare carbon-oxygen cores in tight orbits, with formation rates comparable to or maybe even higher than those of recycled NS-NS binaries. On the other hand, their detection probability as binary pulsars is greatly reduced (by about 1000) relative to recycled pulsars, because of their short lifetimes. We conclude that, in the context of gravitational-wave detection of NS-NS inspiral events, this new type of binaries calls for an increase of the rate estimates derived from the observed NS-NS with recycled pulsars, typically by factors of 1.5-3 or even higher.

astro-ph↗