Gravitational Microlensing Events Due to Stellar‐Mass Black Holes

Abstract

We present an analysis of the longest timescale microlensing events discovered by the MA-CHO Collaboration during a seven year survey of the Galactic bulge. We find six events that exhibit very strong microlensing parallax signals due, in part to accurate photometric data from the GMAN and MPS collaborations. The microlensing parallax fit parameters are used in a likelihood analysis, which is able to estimate the distance and masses of the lens objects based upon a standard model of the Galactic velocity distribution. This analysis indicates that the most likely masses of five of the six lenses are> 1M ⊙ which suggests that a substantial fraction of the Galactic lenses are massive stellar remnants. The lenses for events MACHO-96-BLG-5 and MACHO-98-BLG-6 are the most massive, with mass estimates of M/M ⊙ = 6 +10 −3 and, respectively. The observed upper limits on the absolute brightness of these lenses are < ∼ 1L⊙, so both lenses are black hole candidates. We consider the possibility that the source stars for some of these six events may lie in the foreground Galactic disk or in the Sagittarius (SGR) Dwarf Galaxy behind the bulge, but we find that bulge sources are likely to M/M ⊙ = 6 +7