Hunting gravitational wave black holes with microlensing

Understand

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.