PROBABLE GRAVITATIONAL MICROLENSING TOWARD THE GALACTIC BULGE

The MACHO project carries out regular photometric monitoring of millions of stars in the Magellanic Clouds and the Galactic bulge, to search for very rare gravitational microlensing events due to compact objects in the Galactic halo and disk. A preliminary analysis of one held in the Galactic bulge, containing similar to 430,000 stars observed for 190 days, reveals four stars which show clear evidence for magnifications which are time-symmetric, are achromatic in our two passbands, and have shapes consistent with gravitational microlensing. This is significantly higher than the similar to 1 event expected from microlensing by known stars in the disk. If all four events are due to microlensing, a 95% confidence lower limit on the optical depth toward our bulge field is 1.3 x 10(-6), and a best-fit value is tau approximate to 1.6 x 10(-6)/epsilon, where epsilon is the detection efficiency of the experiment, and epsilon < 0.4. If the true optical depth is close to the best-fit value, possible explanations include a ''maximal'' disk which accounts for most of the Galactic circular velocity at the solar radius, a halo which is centrally concentrated, or bulge-bulge microlensing.