Distances to Galactic X-ray Binaries with Gaia DR2

TL;DR

This study cross-matched Galactic X-ray binaries with Gaia DR2 to measure distances, analyze their spatial distribution relative to spiral arms, and compare different distance estimation methods, improving understanding of XRB properties and locations.

Contribution

It provides the first large-scale Gaia-based distance measurements for X-ray binaries and analyzes their spatial distribution relative to spiral arms, highlighting differences between low- and high-mass XRBs.

Findings

Distances from Gaia are consistent with previous measurements.

Type I X-ray burst distances are systematically larger, indicating they reach only 50% of the Eddington limit.

HMXBs are mildly closer to spiral arms, LMXBs are closer to inter-arm regions.

Abstract

Precise and accurate measurements of distances to Galactic X-ray binaries (XRBs) reduce uncertainties in the determination of XRB physical parameters. We have cross-matched the XRB catalogues of Liu et al. (2006, 2007) to the results of Gaia Data Release 2. We identify 86 X-ray binaries with a Gaia candidate counterpart, of which 32 are low-mass X-ray binaries (LMXBs) and 54 are high-mass X-ray binaries (HMXBs). Distances to Gaia candidate counterparts are, on average, consistent with those measured by Hipparcos and radio parallaxes. When compared to distances measured by Gaia candidate counterparts, distances measured using Type I X-ray bursts are systematically larger, suggesting that these bursts reach only 50% of the Eddington limit. However, these results are strongly dependent on the prior assumptions used for estimating distance from the Gaia parallax measurements. Comparing positions of Gaia candidate counterparts for XRBs in our sample to positions of spiral arms in the Milky Way, we find that HMXBs exhibit mild preference for being closer to spiral arms; LMXBs exhibit mild preference for being closer to inter-arm regions. LMXBs do not exhibit any preference for leading or trailing their closest spiral arm. HMXBs exhibit a mild preference for trailing their closest spiral arm. The lack of a strong correlation between HMXBs and spiral arms may be explained by star formation occurring closer to the midpoint of the arms, or a time delay between star formation and HMXB formation manifesting as a spatial separation between HMXBs and the spiral arm where they formed.