ON THE APPARENT LACK OF Be X-RAY BINARIES WITH BLACK HOLES

In our Galaxy there are 64 Be X-ray binaries known to date. Out of these, 42 host a neutron star (NS), and for the remainder the nature of the companion is unknown. None, so far, are known to host a black hole (BH). There seems to be no apparent mechanism that would prevent formation or detection of Be stars with BHs. This disparity is referred to as a missing Be–BH X-ray binary problem. We point out that current evolutionary scenarios that lead to the formation of Be X-ray binaries predict that the ratio of binaries with NSs to the ones with BHs is rather high, FNStoBH ∼ 10–50, with the more likely formation models providing the values at the high end. The ratio is a natural outcome of (1) the stellar initial mass function that produces more NSs than BHs and (2) common envelope evolution (i.e., a major mechanism involved in the formation of interacting binaries) that naturally selects progenitors of Be X-ray binaries with NSs (binaries with comparable mass components have more likely survival probabilities) over ones with BHs (which are much more likely to be common envelope mergers). A comparison of this ratio (i.e., FNStoBH ∼ 30) with the number of confirmed Be–NS X-ray binaries (42) indicates that the expected number of Be–BH X-ray binaries is of the order of only ∼0–2. This is entirely consistent with the observed Galactic sample.