Gravitational Microlensing Events Due to Stellar‐Mass Black Holes

We present an analysis of the longest timescale microlensing eventsdiscovered by the MACHO Collaboration during a 7 year survey of the Galacticbulge. We find 6 events that exhibit very strong microlensing parallax signalsdue, in part, to accurate photometric data from the GMAN and MPScollaborations. The microlensing parallax fit parameters are used in alikelihood analysis, which is able to estimate the distance and masses of thelens objects based upon a standard model of the Galactic velocity distribution.This analysis indicates that the most likely masses of 5 of the 6 lenses are >1 Msun, which suggests that a substantial fraction of the Galactic lenses maybe massive stellar remnants. This could explain the observed excess of longtimescale microlensing events. The lenses for events MACHO-96-BLG-5 andMACHO-98-BLG-6 are the most massive, with mass estimates of M/Msun = 6 +10/-3and M/Msun = 6 +7/-3, respectively. The observed upper limits on the absolutebrightness of main sequence stars for these lenses are < 1 Lsun, so both lensesare black hole candidates. The black hole interpretation is also favored by alikelihood analysis with a Bayesian prior using a conventional model for thelens mass function. We consider the possibility that the source stars for someof these 6 events may lie in the foreground or background of the Galacticbulge, but we find that this is unlikely. Future HST observations of theseevents can either confirm the black hole lens hypothesis or detect the lensstars and provide a direct measurement of their masses. Future observations ofsimilar events by SIM or the Keck or VLTI interferometers will allow directmeasurements of the lens masses for stellar remnant lenses as well.Comment: 47 pages, with 26 included postscript figures. Includes a new likelihood analysis with a mass function prio