Astrometric excess noise in Gaia EDR3 and the search for X-ray binaries

Astrometric noise (epsilon) in excess of parallax and proper motion is a potential signature of orbital wobble (omega) of individual components in binary star systems. The combination of X-ray selection with astrometric noise could then be a powerful tool for robustly isolating accreting binaries in large surveys. Here, we mine the Gaia EDR3 catalogue for Galactic sources with significant values of astrometric noise over the parameter space expected for known and candidate X-ray binaries (XRBs). Cross-matching our sample with the Chandra Source Catalogue returns a primary sample of approximate to 6500 X-ray sources with significant epsilon. X-ray detection efficiency for objects with significant epsilon is a factor of approximate to 4.5 times higher than in a matched control sample exhibiting low epsilon. The primary sample branches off the main sequence much more than control objects in colour-mag space, and includes a higher fraction of known binaries, variables, and young stellar object class types. However, values of epsilon reported in the Gaia pipeline releases so far can exceed expectations for individual XRBs with known semimajor axis size and other system parameters. It is likely that other factors (possibly attitude and modelling uncertainties, as well as source variability) currently dominate the observed excess noise in such systems. Confirmation of their nature must therefore await future Gaia releases. The full X-ray matched catalogue is released here to enable legacy follow-up.

Automated summary

Astrometric noise combined with X-ray selection is used to identify accreting binaries in large surveys. By cross-matching the Gaia EDR3 catalogue with the Chandra Source Catalogue, a sample of approximately 6500 X-ray sources with significant astrometric noise is obtained. The X-ray detection efficiency for objects with significant noise is about 4.5 times higher than for objects with low noise. These objects, with significant noise, show distinct characteristics in color-magnitude space and include a higher fraction of known binaries, variables, and young stellar objects.