Microlensing optical depth toward the Galactic Bulge using bright sources from OGLE-II

We present a measurement of the microlensing optical depth toward the Galactic Bulge (GB) based on the 4-years OGLE-II survey. Using a sample of 33 events in 5 deg$^2$ GB fields we obtained $\\tau = 1.96_{-0.34}^{+0.41} \\times 10^{-6}$ at $(l,b)=(1.^\\circ16, -2.^\\circ75)$. We considered only bright sources in the extended Red Clump Giant (RCG) region of the Color Magnitude Diagram (CMD), i.e. above the extinction corrected $I$-band magnitude $\\sim15.5$. A gradient along the Galactic latitude $b$ was clearly detected: $\\tau = [ (4.73\\pm 1.74) + (1.02\\pm 0.58)\\times b]\\times 10^{-6}$. The present result, adjusted for the gradient, is consistent with published values based on RCG sources and recent theoretical predictions. Contrary to previous analyses, we do not assume that the apparent RCG sources are free of blending. We found that $\\sim50$% of OGLE-II events which appear to have RCG sources are actually due to much fainter blended stars. This result contradicts a wide-spread belief that the number of spurious RCG events induced by blending is insignificant. Our conclusion is fully supported by extensive Monte-Carlo simulations of the OGLE-II images. We show explicitly that neglecting blends leads to similar optical depth estimates through partial cancellation of contributions from higher detection efficiency, underestimated time-scales and larger number of selected events. Despite this lucky coincidence, strong source confusion effects in typical ground based GB surveys will still bias time-scale distributions and event rates, even for relatively bright stars.

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