THE MACHO PROJECT FIRST-YEAR LARGE MAGELLANIC CLOUD RESULTS - THE MICROLENSING RATE AND THE NATURE OF THE GALACTIC DARK HALO

Since July 1992, the MACHO project has been carrying out long-term pho tometric monitoring of over 20 million stars in the Magellanic Clouds and Galactic bulge. Our aim is to search for the very rare gravitation al microlensing events predicted if the dark halo of our Galaxy is com prised of massive compact halo objects (hereafter MACHOs). We have now analyzed most of the first year's LMC data, comprising 9.5 million li ght curves of stars with an average of 235 observations each. Automate d selection procedures applied to this sample show three events consis tent with microlensing; the first detected is very striking (Alcock an d coworkers), and two are of modest amplitude. We have evaluated our e xperimental detection efficiency using a range of detailed Monte Carlo simulations, including the addition of artificial stars to real data frames. Using a ''standard'' halo density profile, we find that a halo comprised entirely of MACHOs in the mass range 3 x 10(-4) to 0.06 M(. ) would predict more than 15 detected events in this data set, and obj ects around 3 x 10(-3) M(.) would predict 25 events; thus a standard s pherical halo cannot be dominated by objects in this mass range. Assum ing all three events are microlensing by halo objects, and fitting a n aive spherical halo model to our data yields a MACHO halo fraction f = 0.19(-0.10)(+0.16), a total mass in MACHOs (inside 50 kpc) of 7.6(-4) (+6) x 10(10) M(.), and a microlensing optical depth 8.8(-5)(+7) x 10( -8) (68% confidence level). Should only one of these events be microle nsing, this could be explained in terms of previously known population s. We have explored a wide range of halo models and find that, while o ur constraints on the MACHO fraction are quite model dependent, constr aints on the total mass in MACHOs within 50 kpc are quite secure. Futu re observations from this and other similar projects, and accurate mea surements of the Galactic mass out to large radii, should combine to g ive much improved constraints on the MACHO fraction of the halo.