Astrometric excess noise in Gaia DR2 and the search for X-ray emitting
  binaries
  Astrometric noise in excess of parallax and proper motion could be a
signature of orbital wobble of individual components in binary star systems.
The combination of X-ray selection with astrometric noise can then be a
powerful tool for identifying accreting binaries. Here, we mine the Gaia DR2
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 CSC2.0
returns a primary sample of ~1,500 X-ray sources with significant excess noise,
constituting ~0.04% of the initial Gaia sample. By contrast, the fraction of
matched X-ray sources in a control sample with smaller excess noise is a factor
of about 7 lower. The primary sample branches off the main sequence much more
than control objects in colour-mag space, shows a distinct spatial distribution
in terms of Galactic latitudes, comprises more objects with an Halpha excess
and larger X-ray-to-optical flux ratios, and includes a higher fraction of
known binaries, variables and young stellar object class types. However, for
individual XRBs with known system parameters, excess noise can exceed
expectations, especially for small wobble. It is likely that other factors
(possibly attitude and modelling uncertainties, as well as source variability)
currently dominate the observed noise in such systems. Confirmation must
therefore await future Gaia releases. The full Chandra/Gaia matched catalogue
is released here to enable legacy follow-up.
