The Missing Population of Be+Black Hole X-Ray Binaries

At present, 117 Be/neutron star (Be/NS) X-ray binaries (XRBs) are known in the Galaxy and the Magellanic Clouds, but not a single Be/black hole (Be/BH) binary was found so far. We carried out the calculations of stellar population synthesis to investigate the case of the apparently missing population of Be/BH XRBs. According to our calculations, the main reason of this disparity is the fact that within the orbital period range where Be XRBs are found (~10 to ~300 days), these systems are formed predominantly with a NS component. The systems with a BH component are formed predominantly with much longer orbital periods and they are not easy to detect.