Gaia DR2 Distances and Peculiar Velocities for Galactic Black Hole Transients

Poshak Gandhi, Anjali Rao, Michael A. C. Johnson, John A. Paice, T. J. Maccarone, .

Published 2018-04-30Version 1

We report distance estimates for Galactic black hole X-ray binaries with the second data release (DR2) of Gaia, concentrating on sources that are dynamically-confirmed transients or are strong candidates for hosting black holes. While DR1 did not provide parallaxes and proper motions for any of the confirmed black hole binaries, DR2 provides the five-parameter astrometric solutions, including position, parallax and proper motions, for 11 out of a sample of 24 systems, albeit with large uncertainties in some cases. Distance estimates are derived by parallax inversion as well as using Bayesian inference. DR2 distances largely agree with previous estimates. The biggest potential surprise is BW Cir, which DR2 places at a naive (parallax-inversion) distance of ~0.6+/-0.2 kpc, possibly making it the nearest dynamically-confirmed black hole transient. However, Bayesian estimates do allow somewhat larger distances and we caution that correlations between the astrometric parameters need to be accounted for, and that the nearby distance estimate creates problems for the physical interpretation of the properties of the donor star. Under similar caveats regarding the astrometric fit, the super-bright hard-state transient MAXI J1820+070 lies at a likely distance of ~3 kpc. Thus, some care is needed for estimating distances to black hole binaries which typically lie in the faint regime. Peculiar velocities are also computed for 10 sources, and BW Cir could potentially be a new high peculiar-velocity Galactic black hole X-ray binary.