Microlensing events toward the Bulge from MOA-II

For the 111 events in 3.2deg^2^ of the central GB at |b|<=3.{deg}0 and 0.{deg}0<=l<=2.{deg}0, centered at (l,b)=(0.{deg}97, -2.{deg}26), we find {Gamma}=4.57_-0.46_^+0.51^x10^-5^/star/yr and {tau}_200_=3.64_-0.45_^+0.51^x10^-6^. We also consider a red clump giant (RCG) star sample with I_s_<17.5, and we find that the event rate for the RCG sample is slightly lower than but consistent with the all-source event rate. The main difference is the lack of long duration events in the RCG sample due to a known selection effect.

Our results are consistent with previous optical depth measurements, but they are somewhat lower than previous all-source measurements, and slightly higher than previous RCG optical depth measurements. This suggests that the previously observed difference in optical depth measurements between all-source and RCG samples may largely be due to statistical fluctuations. These event rate measurements toward the central GB are necessary to predict the microlensing event rate and to optimize the survey fields in future space missions such as Wide Field Infrared Space Telescope.