The Microlensing Event Rate and Optical Depth from MOA-II 9 Yr Survey Toward the Galactic Bulge

Abstract

Nunota K., Sumi T., Koshimoto N., et al. The Microlensing Event Rate and Optical Depth from MOA-II 9 Yr Survey Toward the Galactic Bulge. Astrophysical Journal 979, 123 (2025); https://doi.org/10.3847/1538-4357/ada352.We present measurements of the microlensing optical depth and event rate toward the Galactic bulge using the data set from the 2006 to 2014 MOA-II survey, which covers 22 bulge fields spanning ∼42 deg2 between −5° < l < 10° and −7° < b < −1°. In the central region with ∣l∣ < 5°, we estimate an optical depth of τ = [ 1.75 ± 0.04 ] × 1 0 − 6 exp [ ( 0.34 ± 0.02 ) ( 3 ∘ − ∣ b ∣ ) ] and an event rate of Γ = [16.08 ± 0.28] × 10−6 exp [ ( 0.44 ± 0.02 ) ( 3 ∘ − ∣ b ∣ ) ] star − 1 yr − 1 using a sample consisting of 3525 microlensing events, with Einstein radius crossing times of tE < 760 days and a source star magnitude of Is < 21.4 mag. We confirm our results are consistent with the latest measurements from the OGLE-IV 8 yr data set. We find our result is inconsistent with a prediction based on Galactic models, especially in the central region with ∣b∣ < 3°. These results can be used to improve the Galactic bulge model, and more central regions can be further elucidated by upcoming microlensing experiments, such as the PRime-focus Infrared Microlensing Experiment and Nancy Grace Roman Space Telescope