Searching for Accreting Compact Object Binaries in SRG/eROSITA eRASS1

Abstract Compact object binaries with accreting white dwarfs, neutron stars, or black holes are crucial for understanding accretion physics. In this study, we identify accreting compact object binary candidates in the Spectrum–Roentgen–Gamma/eROSITA eRASS1 by combining their X-ray fluxes with Gaia photometry and Zwicky Transient Facility (ZTF) time-domain observations. Candidates are selected based on their location in the “X-ray Main Sequence,” a diagram incorporating their X-ray-to-optical flux ratios and optical colors, which suggest accretion-driven X-ray emission. We identify 22 candidates in the eRASS1 catalog using a three-step selection process: (1) cross-matching to a unique Gaia optical counterpart within a 10″ radius; (2) requiring X-ray-to-optical flux ratios exceeding the “X-ray Main Sequence”; and (3) detecting short-period variability in ZTF time-domain photometry. The resulting 22 candidates, including two previously confirmed compact object binaries, represent promising candidates for spectroscopic follow-up to confirm their accreting nature. Our results demonstrate the effectiveness of combining X-ray-to-optical flux ratios and optical colors jointly with time-domain photometry to uncover accreting compact object binaries. The approach is scalable and adaptable to future multiwavelength sky surveys, offering a promising path toward a more complete census of compact object binaries in the Galaxy.