Gaia Data Release 2 distances and peculiar velocities for Galactic black hole transients

We report on a first census of Galactic black hole X-ray binary (BHXRB) properties with the second data release (DR2) of Gaia, focusing on dynamically confirmed and strong candidate BH transients. Gaia DR2 provides five-parameter astrometric solutions including position, parallax and proper motion for 11 out of a sample of 24 systems. Distance estimates are tested with parallax inversion as well as Bayesian inference. We derive an empirically motivated characteristic scalelength of L = 2.17 ± 0.12 kpc for this BHXRB population to infer distances based upon an exponentially decreasing space density prior. Geometric DR2 parallaxes provide new, independent distance estimates, but the faintness of this population in quiescence results in relatively large fractional distance uncertainties. Despite this, DR2 estimates generally agree with the distances found in the literature. The most discrepant case is BW Cir, for which detailed studies of the donor star have suggested a distant location at 25 kpc. The large parallax and relatively high proper motion measured by DR2 instead indicate significantly smaller distances, suggesting that the source might be amongst the nearest of XRBs. However, both larger and smaller distances create problems for the interpretation of the source, and follow-up data are required to resolve its true nature. DR2 also provides a first distance estimate to one source, MAXI J1820+070, and novel proper motion estimates for seven sources. Peculiar velocities relative to Galactic rotation exceed ≈ 50 km s⁻¹ for the bulk of the sample, with a median system kinetic energy of peculiar motion of ∼ 5 × 10⁴⁷ erg. BW Cir could be a new high-velocity BHXRB if its astrometry is confirmed. A putative anticorrelation between peculiar velocity and BH mass is found, as expected in mass-dependent BH kick formation channels, but this trend remains weak in the DR2 data.