Microlensing by Stellar Black Holes around Sagittarius A*

We show that at any given time, the Galactocentric black hole Sgr A* isexpected to be microlensing N(lens)=1.7 bulge stars if the threshold ofdetectability of the fainter image is K(thr)=21, and about N(lens)=8 sources ifK(thr)=23. The lensed images then provide a unique way to detect stellar-massblack holes predicted to cluster around Sgr A*. If a black hole passes close toa microlensed image, it will give rise to a short (weeks long) microlensingevent. We show that the mass and projected velocity of the stellar-mass blackhole can both be measured unambiguously from such an event, provided thateither a caustic crossing is observed or the astrometric displacement ismeasured. For K(thr)=23 and moderate magnifications by Sgr A*, the microlensingevent rate from a cluster of 20000 black holes within a radius of 0.7 pc isonly 0.06 events per year; however, if highly magnified images of a star werefound, the rate of events by the stellar black holes would be much higher. Inaddition, the N(lens) sources lensed by Sgr A* provide a unique probe ofextinction behind the Galactic center along 2N(lens) lines of sight.Comment: 19 pages, 4 figures; replaced with version published in Ap