K-Band Microlensing of the Inner Galaxy

Andrew Gould
1995 Astrophysical Journal  
Microlensing searches toward the inner galaxy (|l|,|b|≤ 22.-2pt'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 time scales 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
more » ... are shorter, the event rate is high Γ∼ 3× 10^-7 day^-1. Fourth, because the Einstein rings are small, r_e∼ 5× 10^12 cm, and the source stars are large r_s 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 ∼ 7 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 2m telescope or four such arrays on a 1m telescope.
doi:10.1086/187933 fatcat:s3jty24qkfaujpgonboy6urghe