K-BAND MICROLENSING OF THE INNER GALAXY

Microlensing searches toward the inner galaxy (\ l \, \ b \ less than or equal to 22'5) have several major advantages. First, the event rate is strongly dominated by bulge-bulge lensing events where both the source and lens lie in the bulge. Second, these bulge-bulge events have very short timescales t(e) similar to 2 days and are therefore easily distinguished from the less frequent and much longer bulge-disk and disk-disk events. Third, since the optical depth is similar to that at higher impact parameters, while the events are shorter, the event rate is high, Gamma similar to 3 x 10(-7) day(-1). Fourth, because the Einstein rings are small, r(e) similar to 5 x 10(12) cm, and the source stars are large, r(s) greater than or similar to 10(12) cm, the lens will transit the face of the source for a significant fraction (similar to 20%) of events. For these transit events it will often be possible to measure a second lens parameter, the angular Einstein radius (or proper motion). In addition to the bulge-bulge events, the optical depth of the disk is similar to 3 times larger toward the inner Galaxy than toward Baade's window. A microlensing search toward the inner Galaxy can be carried out by making frequent (similar to 4 day(-1)) K-band images of a large area similar to 0.5 deg(2) to a depth of K similar to 16 and hence requires either a 1024(2) infrared array on a dedicated 2 m telescope or four such arrays on a 1 m telescope.