AAS 205th Meeting, 9-13 January 2005
Session 32 The Galactic Center
Oral, Monday, January 10, 2005, 10:00-11:30am, Pacific 2/3

## [32.02] Microlensing optical depth towards the Galactic Bulge using bright sources from OGLE-II

T. Sumi (Princeton), P. Wozniak (LANL), A. Udalski, M. Szymanski, M. Kubiak, G. Pietrzynski, I. Soszynski, K. Zebrun, O. Szewczyk, L Wyrzykowski (Warsaw University)

We present a measurement of the microlensing optical depth towards the Galactic Bulge by using bright stars as sources from the central 20 OGLE-II Galactic bulge fields covering a range of 0\circ \circ and -4\circ \circ, the total is about 5 square degrees. From 4 years of data, we detect 61 microlensing candidates on source stars in an Extended Red Clump Giant (RCG) Region in the Colour Magnitude Diagram, where an extinction corrected I-band magnitude is brighter than about 15.5 mag. We find that a half of their source stars which are actually lensed are fainter blended stars. By using the 32 candidates whose actually lensed source stars are still in Extended RCG Region, we estimate the preliminary optical depth \tau ~ 2±0.4 \times 10-6 at (l,b)=(1.16, -2.75) for events with timescales 1< tE <200 days. This value is smaller than previous results with all sources but consistent with previous results with RCG sources and recent theoretical predictions.