Microlensing by Stellar Black Holes around Sagittarius A*

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Abstract
We show that at any given time, the Galactocentric black hole Sgr A* is expected to be microlensing Nlens ~ 1.7 bulge stars if the threshold of detectability of the fainter image is Kthr = 21, and Nlens ~ 8 sources if Kthr = 23. The lensed images then provide a unique way to detect stellar-mass black holes predicted to cluster around Sgr A*. If a black hole passes close to a microlensed image, it will give rise to a short (weeks long) microlensing event. We show that the mass and projected velocity of the stellar-mass black hole can both be measured unambiguously from such an event, provided that either a caustic crossing is observed or the astrometric displacement is measured. For Kthr = 23 and moderate magnifications by Sgr A*, the microlensing event rate from a cluster of 20000 black holes within a radius of 0.7 pc is only 0.06 yr- 1; however, if highly magnified images of a star were found, the rate of events by the stellar black holes would be much higher. In addition, the Nlens sources lensed by Sgr A* provide a unique probe of extinction behind the Galactic center along 2Nlens lines of sight.