Selection effects on the orbital period distribution of low-mass black hole X-ray binaries

Abstract

We investigate the lack of observed low-mass black hole binary systems at short periods(< 4 h) by comparing the observed orbital period distribution of 17 confirmed low-mass black hole X-ray binaries (BHXBs) with their implied period distribution after correcting for the effects of extinction of the optical counterpart, absorption of the X-ray outburst and the probability of detecting a source in outburst. We also draw samples from two simple orbital period distributions and compare the simulated and observed orbital period distributions. We predict that there are > 200 and < 3000 binaries in the Galaxy with periods between 2 and 3 h, with an additional ≈600 objects between 3 and 10 h.