We report the detection of 45 candidate microlensing events in fields toward the Galactic bulge. These come from the analysis of 24 fields containing 12.6 million stars observed for 190 days in 1993. Many of these events are of extremely high signal to noise and are remarkable examples of gravitational microlensing. The distribution of peak magnifications is shown to be consistent with the microlensing interpretation of these events. Using a sub-sample of 1.3 million "Clump Giant" stars whose distance and detection efficiency are well known, we find 13 events and estimate the microlensing optical depth toward the Galactic Bulge as τ bulge = 3.9 +1.8 −1.2 × 10 −6 averaged over an area of ∼ 12 square degrees centered at Galactic coordinates ℓ = 2.55 • and b = −3.64 • . This is similar to the value reported by the OGLE collaboration, and is marginally higher than current theoretical models for τ bulge . The optical depth is also seen to increase significantly for decreasing |b|. These results demonstrate that obtaining large numbers of microlensing events toward the Galactic bulge is feasible, and that the study of such events will have important consequences for the structure of the Galaxy and its dark halo.

The MACHO project carries out regular photometric monitoring of millions of stars in the Magellanic Clouds and 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 eld in the Galactic Bulge, containing 430; 000 stars observed for 190 days, reveals four stars which show clear evidence for brightenings which are time-symmetric, achromatic in our two passbands, and have shapes consistent with gravitational microlensing. This is signi cantly higher than the 1 event expected from microlensing by known stars in the disk. If all four events are due to microlensing, a 95% con dence lower limit on the optical depth towards our bulge eld is 1:3 10 6 , and a \best t" value is 1:6 10 6 = ,where is the detection e ciency of the experiment, and < 0:4. If the true optical depth is close to the \best t" 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.

We present the lightcurves of 21 gravitational microlensing events from the first six years of the MACHO Project gravitational microlensing survey which are likely examples of lensing by binary systems. These events were manually selected from a total sample of ~350 candidate microlensing events which were either detected by the MACHO Alert System or discovered through retrospective analyses of the MACHO database. At least 14 of these 21 events exhibit strong (caustic) features, and 4 of the events are well fit with lensing by large mass ratio (brown dwarf or planetary) systems, although these fits are not necessarily unique. The total binary event rate is roughly consistent with predictions based upon our knowledge of the properties of binary stars, but a precise comparison cannot be made without a determination of our binary lens event detection efficiency. Towards the Galactic bulge, we find a ratio of caustic crossing to non-caustic crossing binary lensing events of 12:4, excluding one event for which we present 2 fits. This suggests significant incompleteness in our ability to detect and characterize non-caustic crossing binary lensing. The distribution of mass ratios, N(q), for these binary lenses appears relatively flat. We are also able to reliably measure source-face crossing times in 4 of the bulge caustic crossing events, and recover from them a distribution of lens proper motions, masses, and distances consistent with a population of Galactic bulge lenses at a distance of 7 +/- 1 kpc. This analysis yields 2 systems with companions of ~0.05 M_sun.Comment: 83 pages, including 5 tables and 48 figures; submitted to The Astrophysical Journal. Data will soon be available at http://wwwmacho.mcmaster.ca/ and http://wwwmacho.anu.edu.au