Microlensing Optical Depth toward the Galactic Bulge from Microlensing Observations in Astrophysics Group Observations during 2000 with Difference Image Analysis

We analyze the data of the gravitational microlensing survey carried out by the Microlensing Observations in Astrophysics (MOA) group during 2000 toward the Galactic bulge (GB). Our observations are designed to detect efficient high-magnification events with faint source stars and short-timescale events, by increasing the sampling rate up to ~6 times per night and using Difference Image Analysis (DIA). We detect 28 microlensing candidates in 12 GB fields corresponding to 16 deg2. We use Monte Carlo simulations to estimate our microlensing event detection efficiency, where we construct the I-band extinction map of our GB fields in order to find dereddened magnitudes. We find a systematic bias and large uncertainty in the measured value of the timescale tE,out in our simulations. They are associated with blending and unresolved sources, and are allowed for in our measurements. We compute an optical depth τ = 2.59 × 10-6 toward the GB for events with timescales 0.3 10). These events are useful for studies of extrasolar planets.