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

TL;DR

This study measures the microlensing optical depth and event rate toward the Galactic bulge using MOA-II data, revealing discrepancies with existing Galactic models especially near the center, and providing data to refine these models.

Contribution

It provides new measurements of microlensing optical depth and event rate over a large bulge area, highlighting inconsistencies with current Galactic models.

Findings

Optical depth and event rate increase towards the Galactic center.

Results are consistent with OGLE-IV measurements.

Discrepancies found with Galactic model predictions in central regions.

Abstract

We present measurements of the microlensing optical depth and event rate toward the Galactic bulge using the dataset from the 2006--2014 MOA-II survey, which covers 22 bulge fields spanning ~42 deg^2 between -5 deg < l < 10 deg and -7 deg < b < -1 deg. In the central region with |l|<5 deg, we estimate an optical depth of {\tau} = [1.75+-0.04]*10^-6exp[(0.34+-0.02)(3 deg-|b|)] and an event rate of {\Gamma} = [16.08+-0.28]*10^-6exp[(0.44+-0.02)(3 deg-|b|)] star^-1 year^-1 using a sample consisting of 3525 microlensing events, with Einstein radius crossing times of tE < 760 days and source star magnitude of IsWe confirm our results are consistent with the latest measurements from OGLE-IV 8 year dataset (Mr\'oz et al. 2019). We find our result is inconsistent with a prediction based on Galactic models, especially in the central region with |b|<3 deg. These results can be used to improve the Galactic bulge model, and more central regions can be further elucidated by future microlensing experiments, such as The PRime-focus Infrared Microlensing Experiment (PRIME) and Nancy Grace Roman Space Telescope.