arXiv subjects

Vassiliki Kalogera

Publications and source records attributed to Vassiliki Kalogera.

68 records · Page 4Linked to original sources

arXiv1999

Compact Binary Mergers and Accretion-Induced Collapse: Event Rates

This paper is a brief review of the topic of binary systems as sources of gravitational-wave emission for both LIGO and LISA. In particular I review the current estimates of the associated Galactic event rates and their implications for expected detection rates. I discuss the estimates for (i) the coalescence of close binaries containing neutron stars or black holes, (ii) white dwarfs going through accretion-induced collapse into neutron stars, and (iii) detached but close binaries containing two white dwarfs. The relevant uncertainties and robustness of the estimates are addressed along with ways of obtaining conservative upper limits.

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arXiv1999

Theoretical Black Hole Mass Distributions

We derive the theoretical distribution function of black hole masses by studying the formation processes of black holes. We use the results of recent 2D simulations of core-collapse to obtain the relation between remnant and progenitor masses and fold it with an initial mass function for the progenitors. We examine how the calculated black-hole mass distributions are modified by (i) strong wind mass loss at different evolutionary stages of the progenitors, and (ii) the presence of close binary companions to the black-hole progenitors. Thus, we are able to derive the binary black hole mass distribution. The compact remnant distribution is dominated by neutron stars in the mass range 1.2-1.6Msun and falls off exponentially at higher remnant masses. Our results are most sensitive to mass loss from winds which is even more important in close binaries. Wind mass-loss causes the black hole distribution to become flatter and limits the maximum possible black-hole mass (<10-15Msun). We also study the effects of the uncertainties in the explosion and unbinding energies for different progenitors. The distributions are continuous and extend over a broad range. We find no evidence for a gap at low values (3-5Msun) or for a peak at higher values (~7Msun) of black hole masses, but we argue that our black hole mass distribution for binaries is consistent with the current sample of measured black-hole masses in X-ray transients. We discuss possible biases against the detection or formation of X-ray transients with low-mass black holes. We also comment on the possibility of black-hole kicks and their effect on binaries.

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arXiv1999

Close Binaries with Two Compact Objects

The coalescence of close binary systems with two compact objects (neutron stars and black holes) are considered to be promising sources of gravitational waves for the currently built laser interferometers. Here, I review the current Galactic coalescence estimates derived both theoretically and empirically. I discuss the uncertainties involved as well as ways of obtaining an upper limit to the coalescence rate of two neutron stars.

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arXiv1999

An Upper Limit on the Coalescence Rate of Double Neutron-Star Binaries in the Galaxy

In the context of assessing the detectability of the coalescence of two neutron stars (NS) by currently built gravitational-wave experiments, we present a way of obtaining an upper limit to the coalescence rate in the Galaxy. We consider the NS/NS progenitors just before the second supernova explosion. By combining our theoretical understanding of orbital dynamics at NS formation with methods of empirically estimating pulsar birth rates we derive an upper limit of a few mergers every 100,000 yr. Such a Galactic rate implies a possible detection by the ``enhanced'' LIGO of up to a few to ten mergers per year.

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arXiv1999

Bounds on Neutron-Star Moments of Inertia and the Evidence for General Relativistic Frame Dragging

Recent X-ray variability observations of accreting neutron stars may provide the first evidence for frame dragging effects around spinning relativistic objects. Motivated by this possibility and its implications for neutron-star structural properties, we calculate new optimal bounds on the masses, radii, and moments of inertia of slowly rotating neutron stars that show kilohertz quasi-periodic oscillations (QPOs). These bounds are derived under minimal assumptions about the properties of matter at high densities and therefore are largely independent of the unknown equation of state. We further derive a semi-analytical upper bound on the neutron-star moment of inertia without making any assumptions about the equation of state of matter at any density. We use this upper bound to show that the maximum possible nodal precession frequency of an inclined circular orbit around a slowly spinning neutron star is nu_{NP}~45.2 (nu_s/300Hz) Hz, where nu_s is the spin frequency of the neutron star. We conclude that the nodal-precession interpretation of low-frequency QPOs in accreting neutron stars is inconsistent with their inferred spin frequencies or the identification of the highest-frequency QPO with that of a circular Keplerian orbit in the accretion disk.

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arXiv1999

Coalescence Rates of Double Neutron Stars

Merger events of close double neutron stars (DNS) lie at the basis of a number of current issues in relativistic astrophysics, such as the indirect and possible direct detection of gravitational waves, the production of gamma-ray bursts at cosmological distances, and the origin of r-process elements in the universe. In assessing the importance or relevance of DNS coalescence to these issues, knowledge of the rate of coalescence in our Galaxy is required. In this paper, I review the current estimates of the DNS merger rate (theoretical and empirical) and discuss new ways to obtain limits on this rate using all information available at present.

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arXiv1999

Donor Stars in Black-Hole X-Ray Binaries

We study theoretically the formation of black-hole (BH) X-ray binaries. Consistency of the models with the observed relative numbers of systems with low-mass (<2 M_sun) and intermediate-mass (~2 M_sun - M_{BH}) donors leads to severe constraints on the evolutionary parameters of the progenitors. In particular, we find that (i) BH progenitor masses cannot exceed about 2 M_{BH}; (ii) high values of the common-envelope efficiency parameter (alpha_{CE} > 1) are required, implying that energy sources other than orbital contraction must be invoked to eject the envelope; (iii) the mass-loss fraction in helium-star winds is limited to be <50%. Outside of this limited parameter space for progenitors we find that either BH X-ray binary formation cannot occur at all or donors do not have the full range of observed masses. We discuss the implications of these results for the structure of massive hydrogen-rich stars, the evolution of helium-stars, and BH formation. We also consider the possible importance of asymmetric kicks.

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arXiv1998

Supernova Kicks, Magnetic Braking, and Neutron-Star Binaries

We consider the formation of low-mass X-ray binaries containing accreting neutron stars via the helium-star supernova channel. The predicted relative number of short-period transients provides a sensitive test of the input physics in this process. We investigate the effect of varying mean kick velocities, orbital angular momentum loss efficiencies, and common envelope ejection efficiencies on the subpopulation of short-period systems, both transient and persistent. Guided by the thermal-viscous disk instability model in irradiation-dominated disks, we posit that short-period transients have donors close to the end of core-hydrogen burning. We find that with increasing mean kick velocity the overall short-period fraction, s, grows, while the fraction, r, of systems with evolved donors among short-period systems drops. This effect, acting in opposite directions on these two fractions, allows us to constrain models of LMXB formation through comparison with observational estimates of s and r. Without fine tuning or extreme assumptions about evolutionary parameters, consistency between models and current observations is achieved for a regime of intermediate average kick magnitudes of about 100-200 km/s, provided that (i) orbital braking for systems with donor masses in the range 1-1.5 solar masses is weak, i.e., much less effective than a simple extrapolation of standard magnetic braking beyond 1.0 solar mass would suggest, and (ii) the efficiency of common envelope ejection is low.

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arXiv1997

Formation of Low-Mass X-Ray Binaries. II. Common Envelope Evolution of Primordial Binaries with Extreme Mass Ratios

We study the formation of low-mass X-ray binaries (LMXBs) through helium star supernovae in binary systems that have each emerged from a common-envelope phase. LMXB progenitors must satisfy a large number of evolutionary and structural constraints, which imposed under the assumption of a symmetric supernova explosion, prohibit the formation of short-period LMXBs transferring mass at sub-Eddington rates through any channel in which the intermediate progenitor of the neutron star is not completely degenerate. Barring accretion-induced collapse, the existence of such systems therefore requires that natal kicks be imparted to neutron stars. We use an analytical method to synthesize the distribution of nascent LMXBs over donor masses and orbital periods, and evaluate their birth rate and systemic velocity dispersion. Within the limitations imposed by observational incompleteness and selection effects, and our neglect of secular evolution in the LMXB state, we compare our results with observations. However, our principal objective is to evaluate how basic model parameters influence these results. We conclude that the characteristics of newborn LMXBs are primarily determined by age and stability constraints and the efficiency of magnetic braking, and are largely independent of the primordial binary population and the evolutionary history of LMXB progenitors (except for extreme values of the average kick magnitude or of the common-envelope ejection efficiency). Theoretical estimates of total LMXB birth rates are not credible, since they strongly depend on the observationally indeterminate frequency of primordial binaries with extreme mass ratios in long-period orbits.

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arXiv1997

Double Neutron Star Systems and Natal Neutron Star Kicks

We study the four double neutron star systems found in the Galactic disk in terms of the orbital characteristics of their immediate progenitors and the natal kicks imparted to neutron stars. Analysis of the effect of the second supernova explosion on the orbital dynamics, combined with recent results from simulations of rapid accretion onto neutron stars lead us to conclude that the observed systems could not have been formed had the explosion been symmetric. Their formation becomes possible if kicks are imparted to the radio-pulsar companions at birth. We identify the constraints imposed on the immediate progenitors of the observed double neutron stars and calculate the ranges within which their binary characteristics (orbital separations and masses of the exploding stars) are restricted. We also study the dependence of these limits on the magnitude of the kick velocity and the time elapsed since the second explosion. For each of the double neutron stars, we derive a minimum kick magnitude required for their formation, and for the two systems in close orbits we find it to exceed 200km/s. Lower limits are also set to the center-of-mass velocities of double neutron stars, and we find them to be consistent with the current proper motion observations.

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arXiv1996

Formation of Low-Mass X-ray Binaries. III. A New Formation Mechanism: Direct Supernova

We propose a new formation mechanism (direct-supernova) for low-mass X-ray binaries (LMXBs) that does not involve any prior phase of mass transfer. Survival through the supernova (SN) explosion and shrinkage of the orbit is achieved by a kick velocity of appropriate magnitude and direction imparted to the neutron star at its birth. We present analytical population synthesis calculations of LMXBs forming via both the direct-SN and the helium-star SN mechanisms, and compare the results. We find that the direct-SN channel contributes a non-negligible fraction of the total LMXB population, depending strongly on the r.m.s. magnitude of the kick velocity. More importantly, the direct-SN mechanism provides a natural way for the formation of low-mass binary pulsars in nearly circular orbits with orbital periods in excess of about 100 days, which cannot have been formed via the helium-star SN mechanism.

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arXiv1996

The Maximum Mass of a Neutron Star

Observational identification of black holes as members of binary systems requires the knowledge of the upper limit on the gravitational mass of a neutron star. We use modern equations of state for neutron star matter, fitted to experimental nucleon-nucleon scattering data and the properties of light nuclei, to calculate, within the framework of Rhoades & Ruffini (1974), the minimum upper limit on a neutron star mass. Regarding the equation of state as valid up to twice nuclear matter saturation density, rho_{nm}, we obtain a secure upper bound on the neutron star mass equal to 2.9 solar masses. We also find that in order to reach the lowest possible upper bound of 2.2 solar masses, we need understand the physical properties of neutron matter up to a density of about 4 times rho_{nm}.

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arXiv1996

Orbital Characteristics of Binary Systems after Asymmetric Supernova Explosions

We present an analytical method for studying the changes of the orbital characteristics of binary systems with circular orbits due to a kick velocity imparted to the newborn neutron star during a supernova explosion (SN). Assuming a Maxwellian distribution of kick velocities we derive analytical expressions for the distribution functions of orbital separations and eccentricities immediately after the explosion, of orbital separations after circularization of the post-SN orbits, and of systemic velocities of binaries that remain bound after the explosion. These distributions of binary characteristics can be used to perform analytical population synthesis calculations of various types of binaries, the formation of which involves a supernova explosion. We study in detail the dependence of the derived distributions on the kick velocity and the pre-SN characteristics, we identify all the limits imposed on the post-SN orbital characteristics, and we discuss their implications for the population of X-ray binaries and double neutron star systems. We show that large kick velocities do not necessarily result in large systemic velocities; for typical X-ray binary progenitors the maximum post-SN systemic velocity is comparable to the relative orbital velocity prior to the explosion. We also find that, unless accretion-induced collapse is a viable formation channel, X-ray binaries in globular clusters have most probably been formed by stellar dynamical interactions only, and not directly from primordial binaries.

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arXiv1995

Formation of Low-Mass X-ray Binaries. I. Constraints on Hydrogen-Rich Donors at the Onset of the X-ray Phase

We identify and quantify the set of constraints that neutron star-normal star binaries must satisfy in order to become observable LMXBs. These constraints are related to (i) the thermal and hydrostatic equilibrium of the donors, (ii) the degree to which the mass transfer process is conservative, and (iii) the age of the systems. They divide the parameter space of potential LMXBs in several distinct parts, of which those that actually become LMXBs at the onset of mass transfer occupy only a small part. Of the remainder, many become unstable to dynamical time scale mass transfer either at the onset or later in the course of mass transfer, and enter common envelope evolution. Others experience super-Eddington mass transfer but may eventually survive to become LMXBs. These survivors arguably include binary millisecond pulsars with orbital periods in excess of $100$\,d, ultrashort-period LMXBs with hydrogen-deficient donors, and long-period LMXBs with giant donors.

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