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

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 21 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 of1100 yr, we infer that the total number of BHXT in the Galaxy is1500. Subject headings: binaries: close—galaxy: kinematics and dynamics—X-rays: general

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