A Search For Stellar-mass Black Holes Via Astrometric Microlensing II: 2012-2015 Keck Candidates
The Milky Way is expected to host $\sim$10$^8$ stellar-mass black holes with an uncertain binary fraction. The only proven method to detect isolated stellar mass black holes is gravitational microlensing. Here we report the results of a microlensing search for black holes with photometry and astrometry. By combining 10 years of seeing-limited photometry from OGLE and MOA with diffraction-limited photometry and astrometry from adaptive optics imagers at the W.~M.~Keck Observatory, we constrain lens masses for OGLE-2012-BLG-0169, OGLE-2014-BLG-0613/MOA-2015-BLG-041, OGLE-2015-BLG-0029/MOA-2015-BLG-170, and OGLE-2015-BLG-0211. Of the four long-duration microlensing events monitored, we ruled out black hole lenses in 3 events, which likely have stellar or white dwarf lenses. OGLE-2015-BLG-0211 remains a black hole candidate with a poorly constrained lens mass with a 1$σ$ upper mass limit of 3.2$M_\odot$ and a 3$σ$ upper mass limit of 21.6$M_\odot$. This event suffered from poor observing conditions and significant astrometric reference frame uncertainties, but its analysis may benefit from additional astrometric data in the upcoming Gaia Data Release 4. Of the six long-timescale ($t_E>100$ days) microlensing events from this work and previous studies, one black hole has been confirmed with a second not ruled out. We briefly examine Galactic model simulations and find that our result agrees with current expectations. Ultimately, we need a larger sample of isolated black holes to constrain their formation processes. This will be possible in the coming years with Rubin and Roman, as well as improved astrometry from JWST and large, ground-based telescopes equipped with adaptive optics.