Of the 10 interacting binaries with strong evidence for a black hole primary, the relatively uniform set of `low mass' systems are particularly important. With mass donors, and mass functions , the accretors are clearly too massive for any realistic neutron star equation of state, and provide the best dynamical evidence for stellar mass black holes. These systems are discovered as X-ray/Optical (X/O) novae and are believed to be disk instability transients. Six of these binaries are now known, discovered over the past 20 years through their X-ray outbursts and confirmed some years after outburst by careful radial velocity studies of their companions in quiescence. Detailed reviews of the observations can be found in Tanaka & Lewin (1995) and Tanaka & Shibazaki (1996); some important properties are reviewed in Sects. 2-4 below. Since the known systems lie at distances of kpc and since the outburst recurrence time seems to be many decades it is clear that many systems remain to be discovered. The goal of this paper will be to extrapolate from the observed sample an estimate of the total population of low mass black hole binaries (BHB) in the Galaxy.
In fact, Tanaka (1992) has already made an order of magnitude estimate of the number of black hole transients in the Galaxy. Updated, the argument is as follows: with sources detected to kpc and y of X-ray sky coverage, the total Galactic population is for an average recurrence time of y. Given that the sky monitoring was not complete and that dynamical measurements of primary mass are only available for a subset of the transient sources, it is likely that this number is a minimal estimate; in this paper attempts are made to improve the completeness corrections. In any case, this large number of BH systems is remarkable, as it exceeds the number of known neutron star low mass X-ray binaries. Such numbers were not entirely unexpected, since Romani (1992) followed common-envelope evolution scenarios similar to those producing neutron star low mass X-ray binaries (LMXB), arguing that with loss of angular momentum determining the accretion rates there should be several hundred short period BH systems in the Galaxy. While the higher mass primaries producing BH should indeed be less common than those producing neutron stars, the key idea in this study was that BH core collapse should be `quiet', leading to small mass loss and small velocity kicks. In contrast to the neutron star case, a large fraction of the initial binaries should remain bound. These computations were extended by Romani (1996), where numbers as large as low mass BH systems and the existence of significant numbers of `intermediate mass' BH systems (with donor mass ) were discussed. However, even with long lifetimes for the BH systems, production of such large numbers through the `He star' core collapse channel requires surprisingly high efficiencies for common envelope ejection. Portegies Zwart, Verbunt and Ergma (1997) for example, find that an application of the standard scenario underproduces the observed low mass BHB by a large factor unless the common envelope ejection efficiency is very high and unless black holes can form from moderate mass progenitors after appreciable narrowing of the orbital system. Large numbers of BH systems are certainly a challenge for evolution scenarios, and an improved census of the black hole population can provide useful constraints on the binary evolution physics.
Here the present sample of X-ray/optical transients is used to improve estimates of the number of Galactic interacting low mass BHB binaries. These computations can be compared with the rate of X-ray transients being discovered by BATSE and similar sky surveys. An estimate is also made of the number of pre-1975 outbursts that went unobserved in X-rays but may have been recorded as interlopers in the sample of classical novae. Prospects for finding more BHB through continued X-ray observations and study of certain classical novae are also discussed. Such data will be very useful in constraining the outburst recurrence time and probing the BHB evolution.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998