The Formation of Black-Hole X-Ray Transients

Studies of the observed characteristics of black-hole (BH) X-ray binaries can be provide us with valuable information about the process of BH formation. In this paper I address some of the aspects of our current understanding of BH formation in binaries and point out some of the existing problems of current theoretical models. In particular, the measured orbital periods and donor-star properties indicate that a common-envelope phase appears to be a necessary ingredient of the evolutionary history of observed BH X-ray transients, and that it must be associated only with a modest orbital contraction. The timing of this common-envelope phase is crucial in determining the final BH masses and current evolutionary models of mass-losing massive stars place strong constraints on the possible masses for immediate BH progenitors and wind mass loss from helium stars. Last, it is interesting that, even in the absence of any source of mass loss, the highest helium-star masses predicted by current evolutionary models are still not high enough to account for the measured BH mass in V404 Cyg (>10 solar masses). An alternative for the formation of relatively massive BH may be provided by the evolutionary sequence proposed by Eggleton & Verbunt (1986), which invokes hierarchical triples as progenitors of BH X-ray binaries with low-mass companions.Comment: 10 pages, 3 figures, to appear in Evolution of Binary and Multiple Star Systems, ASP Conf.Series, 2001, P. Podsiadlowski et al. (eds.) (proceedings from a Meeting in Celebration of Peter Eggleton's 60th Birthday

Event Rates for Binary Inspiral

Double compact objects (neutron stars and black holes) found in binaries with small orbital separations are known to spiral in and are expected to coalesce eventually because of the emission of gravitational waves. Such inspiral and merger events are thought to be primary sources for ground based gravitational-wave interferometric detectors (such as LIGO). Here, we present a brief review of estimates of coalescence rates and we examine the origin and relative importance of uncertainties associated with the rate estimates. For the case of double neutron star systems, we compare the most recent rate estimates to upper limits derived in a number of different ways. We also discuss the implications of the formation of close binaries with two non-recycled pulsars.Comment: 12 pages, to appear in AIP proceedings ``Astrophysical Sources of Gravitational Radiation for Ground-Based Detectors.'

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.Comment: Submitted to ApJ. (v1) Uses emulateapj.cls. 5 figures. (v2): corrected reference list and uses smaller figures (v3): Includes changes in response to referee comments, including new discussion of XRBs. Figures merged, so only 3 figures (v4) Minor typo correction, plus updated abstract posted onlin

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.Comment: 10 pages, uses spie.cls, Proc of the SPIE Conference "Astronomical Telescopes and Instrumentation

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.Comment: 15 pages, 1 figure, to appear in the proceedings ``The influence of binaries on stellar population studies'', Brussels, August 2000 (Kluwer Academic Publishers), ed. D.Vanbevere

Short Gamma-Ray Bursts and Binary Mergers in Spiral and Elliptical Galaxies: Redshift Distribution and Hosts

To test whether the short GRB rates, redshift distribution and host galaxies are consistent with current theoretical predictions, we use avery large database of population synthesis calculations to examine BH-NS and NS-NS merger rates in the universe, factoring in (i) the star formation history of the universe, (ii) a heterogeneous population of star-forming galaxies, including spirals and ellipticals, and (iii) a simple flux-limited selection model for short GRB detection. When we require our models reproduce the known short GRB rates and redshift measurements (and, for NS-NS, the merger rates extrapolated from binary pulsars in the Galaxy), a small fraction of models reproduce all observations, both when we assume a NS-NS and a BH-NS origin for bursts. Most commonly models produce mergers preferentially in spiral galaxies if short GRBs arise from NS-NS mergers alone. Model universes where present-day binary mergers occur preferentially in elliptical galaxies necessarily include a significant fraction of binaries with long delay times between birth and merger (often $O(10{\rm Gyr})$). Though long delays occur, almost all of our models predict that a higher proportion of short GRBs should occur at moderate to high redshift (e.g., $z>1$) than has presently been observed, in agreement with recent observations which suggest a selection bias towards successful follow-up of low-redshift short GRBs. Finally, if only a fraction of BH-NS mergers have the right combination of masses and spins to make GRBs, then at best only a small fraction of BH-NS models could be consistent with all {\em current} available data. (Abridged)Comment: 14 figures, using bitmapped fonts (via eps2eps) to fit in archive space restrictions; better resolution figures are available from the author. Accepted for publication in ApJ. v3 updates reference

Formation of low-mass x-ray binaries; 1, 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

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.Comment: (v1) Submitted to ApJ. Uses emulateapj.cls. 8 pages, 7 figures. (v2) Minor textual changes in response to referee queries. Substantial additions in appendicies, including a detailed discussion of sample multidimensional population synthesis fit