Program: GN-2020B-Q-318

Our Milky Way is probably full of stellar-mass black holes, but so far only about 50 are known, and all are in X-
ray binaries. Gravitational microlensing is the only tool capable of detecting single black holes, which are not

interacting with anything. However, despite of more than 15,000 microlensing events found to date, we are not

able to tell which ones are due to black holes. Combination of photometry from the ground and astrometric time-
series from Gaia will yield mass measurements for the longest lasting events, but without the lens and source

distances, the degeneracy between massive stars and black holes still remains. We propose to conduct Standard
ToO GMOS observations of selected microlensing events from the Galactic plane outside of the bulge, for which
Gaia will provide astrometric data and which are good candidates for lensing black holes, in order to constrain the
spectral type of their sources and any blends.