Near-Infrared Microlensing of Stars by the Super-Massive Black Hole in the Galactic Center

We investigate microlensing amplification of faint stars in the dense stellar cluster in the Galactic Center by the super-massive black hole, which is thought to coincide with the radio source SgrA . Such amplification events would appear very close to the position of SgrA , and could be observed, in principle, during the monitoring of stellar proper motions in the Galactic Center. We use observations of the near-infrared K-band (2.2μm) luminosity function in the Galactic Center and in Baade’s Window, as well as stellar population synthesis computations, to construct empirical and theoretical K luminosity function models for the inner 300 pc of the Galaxy. These, together with the observed dynamical properties of the central cluster and inner bulge, are used to compute the rates of microlensing events which amplify stars with different intrinsic luminosities above specified detection thresholds. We present computations of the lensing rates as functions of the event durations, which range from several weeks to a few years, for detection thresholds ranging from K0 = 16 m to 19m. We find that events with durations shorter than a few months dominate the lensing rate because of the very high stellar densities and velocities very near the black hole, where the effective lens size is small. For the current detection limit of K0 = 17 m, the total microlensing rate is 3× 10−3 yr−1. The rate of events with durations ≥ 1 yr is 1× 10−3 yr−1. The median value of the peak amplification for short events is ∆K ∼ 0.75m above the detection threshold, and is only weakly dependent on K0. Long events are rarer, and are associated with more distant stars, stars at the low velocity tail of the velocity distribution, or stars that cross closer to the line-of-sight to E-mail address: [email protected] E-mail address: [email protected] On leave at the Department of Astronomy, University of California, Berkeley, CA 94720