Microlensing by Stellar Black Holes Around Sgr A*

Abstract

We show that at any given time, the Galactocentric black hole Sgr A* is expected to be microlensing N(lens)=1.7 bulge stars if the threshold of detectability of the fainter image is K(thr)=21, and about N(lens)=8 sources if K(thr)=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 K(thr)=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 events per year; 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 N(lens) sources lensed by Sgr A* provide a unique probe of extinction behind the Galactic center along 2N(lens) lines of sight.