Finding Black Holes with Microlensing

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Abstract
The MACHO and OGLE collaborations have argued that the three longest-duration bulge microlensing events are likely caused by nearby black holes, given the small velocities measured with microlensing parallax and non-detection of the lenses. However, these events may be due to lensing by more numerous lower-mass stars at greater distances. We find a-posteriori probabilities of 95%, 58%, and 34% that the three longest events are black holes, assuming a Salpeter IMF and 40 solar mass cutoff for neutron-star-progenitors; the numbers depend strongly on the assumed mass function, but favor a black hole for the longest event for any standard IMF. The longest events (> 600 days) have an a-priori ~26% probability to be black holes for a standard mass function. We propose a new technique for measuring the lens mass function using the mass distribution of long events measured with ACS, VLTI, SIM, or GAIA.

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