On the apparent lack of be x-ray binaries with black holes in the galaxy and in the Magellanic clouds

TL;DR

The paper investigates the absence of observed Be X-ray binaries with black holes in the galaxy and Magellanic Clouds, analyzing population synthesis models and star formation histories to explain this discrepancy.

Contribution

It provides a detailed analysis of the missing Be+BH X-ray binaries problem, highlighting the importance of star formation history differences between the galaxy and Magellanic Clouds.

Findings

Galactic Be+BH X-ray binaries expected: 0-2, consistent with observations.

Magellanic Clouds expected Be+BH X-ray binaries: about 6, but none observed.

Star formation history differences may explain the discrepancy.

Abstract

In the Galaxy and in the Magellanic Clouds there are 170 Be X-ray binaries known to-date. Out of those, 111 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. This disparity is referred to as a missing Be -- black hole X-ray binary problem. The stellar population synthesis calculations following the formation of Be X-ray binaries in the Galaxy (Belczynski and Ziolkowski 2009) demonstrate that there is no problem of the missing Be+BH X-ray binaries for the Galaxy (the expected number of Be -- black hole X-ray binaries is 0 to 2, which is entirely consistent with the observed Galactic sample). However, the preliminary calculations for Magellanic Clouds indicate that there is a problem of the missing Be+BH X-ray binaries for the Clouds (the expected number is about 6, while none is observed). We believe, that to remove the discrepancy, one has to take into account a different history of the star formation rate in the Magellanic Clouds, with the respect to the Galaxy. New stellar population synthesis calculations are currently being carried out. An updated (as of November 2011) list of all 170 Be X-ray binaries known presently in the Galaxy and in the Magellanic Clouds is included.