The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the Ðrst 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected (D1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with days to be based upon our eight event sample. This exceeds the 2 \ tü \ 200 q 2 200 \ 2.9~0 .9 1.4 ] 10~7 optical depth, expected from known stars, and the di †erence is to be compared q backgnd \ 0.5 ] 10~7, with the optical depth predicted for a "" standard ÏÏ halo composed entirely of MACHOs : q halo \ 4.7 To compare with Galactic halo models, we perform likelihood analyses on the full eight-event ] 10~7. sample and a six-event subsample (which allows for two events to be caused by a nonhalo "" background ÏÏ). This gives a fairly model-independent estimate of the halo mass in MACHOs within 50 kpc of which is about half of the "" standard halo ÏÏ value. We also Ðnd a most prob-2.0~0 .71.2 ] 1011 M _ , able MACHO mass of although this value is strongly model dependent. In addition, the 0.5~0 .2 0.3 M _ , absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs : objects from 10~4 to 0.03 contribute of the "" standard ÏÏ dark halo.

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.

No abstract