On the Apparent Lack of Be X-ray Binaries with Black Holes

Abstract

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 ref erred 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 FNStoBH ∼ 30 − 50. The ratio is a natural outcome of (i) the stellar initial mass function that provides 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 (comparable mass binaries have more likely survival probabilities) against ones with black hol es (much more likely common envelope mergers). A comparison of this ratio (i.e. FNStoBH ∼ 30 − 50) with the number of confirmed Be ‐ neutron star X-ray binaries (42) indicates that the expect ed 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.