On the Apparent Lack of Be X-ray Binaries with Black Holes
  In the Galaxy there are 64 Be X-ray binaries known to-date. Out of those, 42
host a neutron star, and for the reminder the nature of a companion is not
known. None, so far, is known to host a black hole. There seems to be no
apparent mechanism that would prevent formation or detection of Be stars with
black holes. This disparity is referred to as a missing Be -- black hole 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 these binaries
with neutron stars to the ones with black holes is rather high F_NStoBH=10-50,
with the more likely formation models providing the values at the high end. The
ratio is a natural outcome of (i) the stellar initial mass function that
produces more neutron stars than black holes and (ii) 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 neutron stars (binaries
with comparable mass components have more likely survival probabilities) over
ones with black holes (which are much more likely to be common envelope
mergers). A comparison of this ratio (i.e. F_NStoBH=30) with the number of
confirmed Be -- neutron star X-ray binaries (42) indicates that the expected
number of Be -- black hole X-ray binaries is of the order of only 0-2. This is
entirely consistent with the observed Galactic sample.
