Quiescent and Interacting Compact Remnant Binary Populations in the Space Telescope Era

Abstract: Since the discovery of Gaia BH1, interest in quiescent binary systems has grown. Two additional black-hole binaries have since been discovered alongside several candidate neutron-star binaries, yet the formation pathways of these systems remain poorly understood. Theoretical studies of these systems, together with the populations into which they evolve, such as X-ray binaries (XRBs), can help constrain their formation channels. We present predictions for the Galactic population of quiescent binary systems and their later evolutionary phases, including XRBs and post-interaction quiescent binaries. We combine stellar models from the Binary Population and Spectral Synthesis code (BPASS) with a synthetic Milky Way generated using Feedback In Realistic Environments (FIRE) simulations. We compare our results with existing observations to assess how well the synthetic population reproduces the observations. We find generally good agreement, with observed donor masses, compact-object masses, and orbital periods largely reproduced, except for the Gaia BH systems. Our results also suggest that some accreting compact remnants with low-mass donors and wide orbital periods may not be X-ray bright. Remaining discrepancies indicate that the BPASS initial parameter space should be expanded, the common-envelope evolution prescription revised, and magnetic braking included in low-mass donor models. We also predict the number of systems detectable in future Gaia data releases and by the Nancy Grace Roman Space Telescope (hereafter referred to as Roman). In particular, the Galactic Bulge Time Domain Survey is expected to yield a significant population of quiescent black-hole binaries. We predict that ~200-600 candidate quiescent binary systems will be detected in the Galactic Bulge Time Domain Survey, while ~2500-4000 candidate systems will be identified in Gaia DR5.