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

Abstract

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 black hole transients. DR2 provides five-parameter astrometric solutions including position, parallax and proper motion for 11 of a sample of 24 systems. Distance estimates are tested with parallax inversion as well as Bayesian inference. We derive an empirically motivated characteristic scale length 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 literature distances. The most discrepant case is BW Cir, for which detailed studies of the donor star have suggested a distant location at ∼ > 25 kpc. A large DR2 measured parallax and relatively high proper motion instead prefer significantly smaller distances, suggesting that the source may instead be amongst the nearest of XRBs. However, both distances create problems for 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 7 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 × 10 47 erg. BW Cir could be a new high-velocity BHXRB if its astrometry is confirmed. A putative anti-correlation between peculiar velocity and black hole mass is found, as expected in mass-dependent BH kick formation channels, but this trend remains weak in the DR2 data.

A radio parallax to the black hole X-ray binary MAXI J1820+070

“…The mode and the 68 per cent higher density interval values (Bailer-Jones 2015) of this distribution are 4.13 +2.57 −1.5 kpc. We also tested an exponentially decreasing volume density (EDVD) prior (Bailer-Jones 2015) using the scaling length parameter L for BHXBs from Gandhi et al (2019) and find that unlike the Gaia-DR2 distance posterior distribution (median of 4.4 ± 2.4 kpc; mode of 3.46 +2.13 −1.06 kpc), our new distance measurement is unaffected by the chosen prior (see Figure 2). This shows that the high significance of our parallax measurement has led to a model-independent distance to the source.…”

MAXI J1820+070 with NuSTAR I. An increase in variability frequency but a stable reflection spectrum: coronal properties and implications for the inner disc in black hole binaries

“…The distance to MAXI J1820+070 is still moderately uncertain. Among the first set of XRB distances to be derived directly from optical astrometry (rather than indirect photometric and spectroscopic methods), Gaia measurements of the system in quiescence give a parallax of 0.31 ± 0.11 milliarcsec, which corresponds to a distance of 3.5 +2.2 −1.0 kpc (Gandhi et al 2019). This should be improved in the next Gaia data release, especially considering the long interval over which the source remained bright.…”

Potential kick velocity distribution of black hole X-ray binaries and implications for natal kicks

“…We use the likelihood function as described in Bailer-Jones (2015) and Gandhi et al (2019). We follow the work by Grimm et al (2002, GR02 hereafter) to make an analytical model of the LMXB density distribution in our Galaxy to use as the prior.…”