The Galactic Distribution of Black Hole Candidates in Low-Mass X-Ray Binary Systems

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Abstract
We find that for black hole candidates (BHC) in low-mass X-ray binaries (LMXRB) the dispersion of their distances, z, to the Galactic plane is smaller than for LMXRB with neutron stars by more than a factor 2. This result indicates that any kick velocities that the black hole systems may have received at formation are substantially smaller than those of neutron star systems. We infer a value of 40 km s-1 for the radial-velocity dispersion of the BHC systems from their z dispersion, consistent with the value obtained directly from radial-velocity measurements. From the surface density of black hole X-ray transients (BHXT) in the solar vicinity, and an estimate for the typical recurrence time of ~100 yr, we infer that the total number of BHXT in the Galaxy is ~500.