Enhanced Microlensing by Stars around the Black Hole in the Galactic Center

The effect of stars on the lensing properties of the supermassive black holein the Galactic Center is similar to the effect of planets on microlensing by astar. We show that the dense stellar cluster around SgrA* increases by factorsof a few the probability of high-magnification lensing events of a distantbackground source by the black hole. Conversely, the gravitational shear of theblack hole changes and enhances the microlensing properties of the individualstars. The effect is largest when the source image lies near the Einsteinradius of the black hole 1.75"+/-0.20" for a source at infinity). We estimatethat the probability of observing at least one distant background star which ismagnified by a factor >5 in any infrared snapshot of the inner ~ 2" of theGalactic Center is 1% with a K-band detection threshold of 20 mag. The largestsource of uncertainty in this estimate is the luminosity function of thebackground stars. The gravitational shear of the black hole lengthens theduration of high-magnification events near the Einstein radius up to a fewmonths, and introduces a large variety of lightcurve shapes that are differentfrom those of isolated microlenses. Identification of such events by imagesubtraction can be used to probe the mass function, density and velocitydistributions of faint stars near the black hole, which are not detectableotherwise.Comment: 16 pages, 4 figures, ApJ in press. Lensing probability for GC significantly decreased due to revised background sources mode