Black Hole Astrometric Binaries in the Roman Galactic Bulge Time Domain Survey

Abstract The Nancy Grace Roman Space Telescope (Roman), NASA’s newest flagship mission, launched 2026 August 30. As part of its mission, Roman will conduct the Galactic Bulge Time Domain Survey (GBTDS), which will generate ∼50,000 epochs of high-precision photometric and astrometric data for ∼10 8 sources across 1.7 deg 2 in the Galactic bulge. Roman GBTDS astrometry is comparable to Gaia Data Release 4 in terms of number of stars, astrometric precision, and time baseline, and is highly complementary in terms of wavelength and sky location. In this paper, we investigate a synthetic population of GBTDS sources to characterize the detectability of unresolved astrometric binaries, in particular those with compact object companions. Assuming the occurrence rate of black holes (BHs) and neutron stars (NSs) in astronomical-unit-scale orbits around stars is 10 −7 and 10 −6 , respectively, and that Roman achieves an astrometric precision of 1% of a pixel, O ( 10 ) BH+star and O ( 10 ) NS+star detached binaries will be detectable. The BHs will have median mass measurement uncertainties of ∼25%, increasing the existing sample of detached astrometric BH binaries by a factor of 3. Together with the O ( 1 0 2 ) isolated BHs expected to be discovered by microlensing in the Roman GBTDS and an additional O ( 10 ) detached BH binaries in Gaia DR4, this will provide a representative view of the quiescent Galactic stellar-mass BH population.