On the Possibility of Stellar Lenses in the Black Hole Candidate
  Microlensing Events MACHO-96-BLG-5 and MACHO-98-BLG-6
  Though stellar-mass black holes (BHs) are likely abundant in the Milky Way
(N=10^8-10^9), only ~20 have been detected to date, all in accreting binary
systems (Casares 2006). Gravitational microlensing is a proposed technique to
search for isolated BHs, which to date have not been detected. Two microlensing
events, MACHO-1996-BLG-5 (M96-B5) and MACHO-1998-BLG-6 (M98-B6), initially
observed near the lens-source minimum angular separation in 1996 and 1998,
respectively, have long Einstein crossing times (>300 days), identifying the
lenses as candidate black holes. Twenty years have elapsed since the time of
lens-source closest approach for each of these events, indicating that if the
lens and source are both luminous, and if their relative proper motion is
sufficiently large, the two components should be spatially resolvable. We
attempt to eliminate the possibility of a stellar lens for these events by: (1)
using Keck near-infrared adaptive optics images to search for a potentially
now-resolved, luminous lens; and (2) examining multi-band photometry of the
source to search for flux contributions from a potentially unresolved, luminous
lens. We combine detection limits from NIRC2 images with light curve data to
eliminate all non-BH lenses for relative lens-source proper motions above 0.81
mas/yr for M96-B5 and 2.48 mas/yr for M98-B6. Further, we use WFPC2 broadband
images to eliminate the possibility of stellar lenses at any proper motion. We
present the narrow range of non-BH possibilities allowed by our varied
analyses. Finally, we suggest future observations that would constrain the
remaining parameter space with the methods developed in this work.
