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Astron. Astrophys. 333, 583-590 (1998)

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7. Conclusions

In this paper, the X/O novae dynamically confirmed as [FORMULA] black hole candidates with low mass secondaries have been studied as representatives of the Galactic source population. This relatively homogeneous class seems to be a dominant component of the Galactic X-ray transients. It was shown that reasonable recurrence times for these systems can be derived by assuming the disk instability model with mass transfer driven by nuclear evolution of the secondary or by gravitational radiation loss of orbital angular momentum. Magnetic braking of the form commonly assumed for close binaries does not seem to operate in these systems, at least for [FORMULA].

An extrapolation to the Galactic population of such systems confirms simple estimates, giving under fairly conservative assumptions a total Galactic number of [FORMULA] low mass BHB that appear as X/O novae. No one system dominates the extrapolated population numbers, so this estimate is relatively immune to uncertainties in the parameters or evolution of any one observed system. Plausible changes to the model assumptions may allow numbers to range from [FORMULA]. This substantially exceeds the number of known neutron star LMXB ([FORMULA] 200), most of which are persistent. There are several transient neutron star LMXB known (Tanaka & Lewin 1995, Tanaka & Shibazaki 1996). Excluding the large number of Galactic Be X-ray binaries, it seems likely that the transients will not dominate the neutron star LMXB population, both from evolutionary arguments (King & Ritter 1997) and from the smaller numbers, larger typical distance and shorter recurrence time of the detected neutron star transients compared to the BH X/O novae. A quantitative assesment of the transient contribution to the neutron star LMXB population of the sort made here for the BH systems is needed, but it seems that even including these systems there will be appreciably more BH LMXB than those containing neutron stars. Given the expectation that BH progenitors are much rarer, this presents a challenge for models of binary evolution and may place particular pressure on scenarios forming high mass ratio binaries after a common envelope phase.

The modeled X/O nova rate provides good agreement with the BATSE detection rate of X-ray transients. BATSE's hard band sensitivity prevents it from seeing most UP systems. In fact, hard spectrum objects like 0422+32 should dominate BATSE's future detections, giving a rather biased view of the BHB population. Nevertheless, follow-on radial velocity studies of X/O novae in quiescence, requiring in many cases 10m-class telescopes, should produce a steadily increasing number of sources of this class.

A similar prediction can be made for the occurrence of optical detections of X/O novae in classical nova catalogues; [FORMULA] should be present in compilations of novae recorded before 1975 in the pre-X-ray survey era. Any recovery of a BH binary from a historical nova will be particularly valuable for constraining the outburst recurrence time and the physics of the accretion disk during quiescence. More detailed study of the archive plates would, of course, turn up many more BH candidate systems. Searches of the historical novae will also be especially interesting as the selected systems will tend to be relatively bright, and hence nearby (although a possible bias towards large [FORMULA] binaries with evolved companions has been noted; roughly half of the systems with outburst magnitudes [FORMULA] are expected to be similar to 2023+338). The exciting possibility of discovering a system substantially closer than the closest known BH (A0620-003) is a further reason to search for historical X/O novae.

For a total population of 1670 systems, the closest example should be at [FORMULA] kpc. With an outburst frequency-weighted absolute magnitude in quiescence of [FORMULA], such a system would appear at [FORMULA]. The average (nearest) systems will have long recurrence times, so only [FORMULA] % would be present in historical nova catalogues. Nevertheless, the nearest recovered system would then be relatively close and bright at [FORMULA] kpc, even disregarding the optical bias towards recovering the brightest nearby outbursts. The possibility of finding many more BH binary systems, including close, bright examples and eclipsing systems, raises the hope that one may be able to subject newly detected black hole binaries to a number of high precision studies. The prospect of closely studying the physics of the accretion disk and even of probing the space-time metric near the central holes themselves (eg. Zhang et al. 1997) makes the future study of low mass black hole binaries particularly appealing.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998