A comprehensive search for high-velocity X-ray sources: new compact object binary candidates in the Gaia era

ABSTRACT We perform a comprehensive search for high-velocity X-ray sources with large X-ray/optical flux ratios ($F_\mathrm{X}/F_\mathrm{G}$), identifying candidates for interacting black hole or neutron star binaries potentially accelerated by supernova natal kicks. We cross-match X-ray points sources from a variety of catalogues (Chandra, XMM–Newton, Swift, and eROSITA) with Gaia DR3. Using Gaia coordinates, parallaxes, and proper motions, we compute peculiar velocities ($\upsilon _\mathrm{pec}$) relative to Galactic disc rotation. Remaining agnostic about radial velocities (RVs), we vary RVs to find the minimum possible $\upsilon _\mathrm{pec}$ values ($\upsilon _\mathrm{pec, min}$). Uncertainties on $\upsilon _\mathrm{pec, min}$ are estimated via Monte Carlo resampling, and we select X-ray sources that have $1\, \sigma$ lower limits on $\upsilon _\mathrm{pec, min}\ge 200\, \mathrm{km~s^{-1}}$ and high $F_\mathrm{X}/F_\mathrm{G}$ values. We show that this velocity threshold excludes most contaminants (e.g. cataclysmic variables and active binaries) while retaining a sensible fraction of compact object binaries, demonstrating that $\upsilon _\mathrm{pec}$ could serve as an effective indicator for the presence of a neutron star or black hole companion. Our selection yields a sample of $2372$ sources, from which we construct a gold sample of $7$ sources that have relatively well-constrained astrometry and confident optical counterparts. Follow-up is necessary to confirm and characterise their high-energy emission, as well as a Galactic disc vs. halo origin.