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While dozens of stellar mass black holes have been discovered in binarysystems, isolated black holes have eluded detection. Their presence can beinferred when they lens light from a background star. We attempt to detect theastrometric lensing signatures of three photometrically identified microlensingevents, OGLE-2011-BLG-0022, OGLE-2011-BLG-0125, and OGLE-2012-BLG-0169(OB110022, OB110125, and OB120169), located toward the Galactic Bulge. Theseevents were selected because of their long durations, which statisticallyfavors more massive lenses. Astrometric measurements were made over 1-2 yearsusing laser-guided adaptive optics observations from the W. M. KeckObservatory. Lens model parameters were first constrained by the photometriclight curves. The OB120169 light curve is well-fit by a single-lens model,while both OB110022 and OB110125 light curves favor binary-lens models. Usingthe photometric fits as prior information, no significant astrometric lensingsignal was detected and all targets were consistent with linear motion. Thesignificant lack of astrometric signal constrains the lens mass of OB110022 to0.05-1.79 Msun in a 99.7% confidence interval, which disfavors a black holelens. Fits to OB110125 yielded a reduced Einstein crossing time andinsufficient observations during the peak, so no mass limits were obtained. Twodegenerate solutions exist for OB120169, which have a lens mass between0.2-38.8 Msun and 0.4-39.8 Msun for a 99.7% confidence interval. Follow-upobservations of OB120169 will further constrain the lens mass. Based on ourexperience, we use simulations to design optimal astrometric observingstrategies and show that, with more typical observing conditions, detection ofblack holes is feasible.

A Search for Stellar-Mass Black Holes via Astrometric Microlensing