While dozens of stellar mass black holes have been discovered in binary systems , isolated black holes have eluded detection .
Their presence can be inferred when they lens light from a background star .
We attempt to detect the astrometric lensing signatures of three photometrically identified microlensing events , OGLE-2011-BLG-0022 , OGLE-2011-BLG-0125 , and OGLE-2012-BLG-0169 ( OB110022 , OB110125 , and OB120169 ) , located toward the Galactic Bulge .
These events were selected because of their long durations , which statistically favors more massive lenses .
Astrometric measurements were made over 1–2 years using laser-guided adaptive optics observations from the W. M. Keck Observatory .
Lens model parameters were first constrained by the photometric light curves .
The OB120169 light curve is well-fit by a single-lens model , while both OB110022 and OB110125 light curves favor binary-lens models .
Using the photometric fits as prior information , no significant astrometric lensing signal was detected and all targets were consistent with linear motion .
The significant lack of astrometric signal constrains the lens mass of OB110022 to 0.05–1.79 M _ { \odot } in a 99.7 % confidence interval , which disfavors a black hole lens .
Fits to OB110125 yielded a reduced Einstein crossing time and insufficient observations during the peak , so no mass limits were obtained .
Two degenerate solutions exist for OB120169 , which have a lens mass between 0.2–38.8 M _ { \odot } and 0.4–39.8 M _ { \odot } for a 99.7 % confidence interval .
Follow-up observations of OB120169 will further constrain the lens mass .
Based on our experience , we use simulations to design optimal astrometric observing strategies and show that , with more typical observing conditions , detection of black holes is feasible .