2020ApJ...905..121A

Gravitational microlensing is a powerful tool to search for a population of invisible black holes (BHs) in the Milky Way (MW), including isolated BHs and binary BHs at wide orbits that are complementary to gravitational wave observations. By monitoring highly populated regions of source stars like the MW bulge region, one can pursue microlensing events due to these BHs. We find that if BHs have a Salpeter-like mass function extended beyond 30M_☉ and a similar velocity and spatial structure to stars in the Galactic bulge and disk regions, the BH population is a dominant source of microlensing events at long timescales of the microlensing light curve >=100 days. This is due to a boosted sensitivity of the microlensing event rate to lens mass, given as M², for such long-timescale events. A monitoring observation of 2 x 10¹⁰ stars in the bulge region over 10 yr with the Rubin Observatory Legacy Survey of Space and Time (LSST) would enable one to find about 6 x 10⁵ BH microlensing events. We evaluate the efficiency of potential LSST cadences for characterizing the light curves of BH microlensing and find that nearly all events of long timescales can be detected.