K-BAND MICROLENSING OF THE INNER GALAXY

摘要

Microlensing searches toward the inner galaxy (|l|, |b| ≤ 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 ～ 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, Γ ～ 3 x 10~(-7) day~(-1). Fourth, because the Einstein rings are small, r_e～5x 10~(12) cm, and the source stars are large, r_s approx > 10~(12) cm, the lens will transit the face of the source for a significant fraction (～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 ～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 (～4 day~(-1)) K-band images of a large area ～0.5 deg~2 to a depth of K ～ 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.