Hunting gravitational wave black holes with microlensing

TL;DR

Microlensing surveys, especially with LSST, can detect a large population of otherwise invisible black holes in the Milky Way, including isolated and wide-orbit binary black holes, by monitoring long-duration events.

Contribution

This study demonstrates that long-timescale microlensing events are highly sensitive to black hole populations, proposing LSST observations as a powerful method to detect and characterize these black holes.

Findings

LSST could detect approximately 600,000 black hole microlensing events over 10 years.

Long-timescale microlensing events are predominantly caused by black holes with a Salpeter-like mass function.

Nearly all long-duration events can be effectively detected with LSST cadences.

Abstract

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 $30M_\odot$ and a similar velocity and spatial structure to stars in the Galactic bulge and disk regions, the BH population is a dominant source of the microlensing events at long timescales of the microlensing light curve $\gtrsim 100~$days. This is due to a boosted sensitivity of the microlensing event rate to lens mass, given as $M^2$, for such long-timescale events. A monitoring observation of $2 \times 10^{10}$ stars in the bulge region over 10 years with the Rubin Observatory Legacy Survey of Space and Time (LSST) would enable one to find about $6\times 10^5$ 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.