The MACHO project: Microlensing Optical Depth towards the Galactic Bulge from Difference Image Analysis

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  • 29 February 2000
Abstract
We present the microlensing optical depth towards the Galactic bulge based on the detection of 99 events found in our Difference Image Analysis (DIA) survey. This analysis encompasses three years of data, covering ~ 17 million stars in ~ 4 sqdeg, to a source-star baseline magnitude limit of V = 23. The DIA technique improves the quality of photometry in crowded fields, and allows us to detect more microlensing events with faint source stars. We find this method increases the number of detection events by 85% compared with the standard analysis technique. DIA light curves of the events are presented and the microlensing fit parameters are given. The total microlensing optical depth is estimated to be tau(total)= 2.91(+0.47/-0.45) x 10^(-6) averaged over 8 fields centered at l=2.68 deg and b= -3.35 deg. For the bulge component we find tau(bulge) = 3.88(+0.63/-0.60) x 10^(-6) assuming a 25% contribution from disk microlensing events. These optical depths are in good agreement with the past determinations of the MACHO Alcock et al. (1997a) and OGLE Udalski et al. (1994) groups, and are significantly higher than predicted by contemporary Galactic models. We show that the short end of microlensing event timescale distribution is compatible with that expected from the mass distribution of normal stars. We thus find no evidence for the existence of a large population of brown dwarfs in the direction of the Galactic bulge.

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