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 photometric monitoring of over 20 million stars in the Magellanic Clouds and Galactic bulge. Our aim is to search for the very rare gravitational microlensing events predicted if the dark halo of our Galaxy is comprised of massive compact halo objects (hereafter MACHOs). We have now analyzed most of the first year's LMC data, comprising 9.5 million light curves of stars with an average of 235 observations each. Automated selection procedures applied to this sample show three events consistent with microlensing; the first detected is very striking (Alcock and coworkers), and two are of modest amplitude. We have evaluated our experimental 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 objects around 3 x 10(-3) M(.) would predict 25 events; thus a standard spherical halo cannot be dominated by objects in this mass range. Assuming all three events are microlensing by halo objects, and fitting a naive 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 microlensing, this could be explained in terms of previously known populations. We have explored a wide range of halo models and find that, while our constraints on the MACHO fraction are quite model dependent, constraints on the total mass in MACHOs within 50 kpc are quite secure. Future observations from this and other similar projects, and accurate measurements of the Galactic mass out to large radii, should combine to give much improved constraints on the MACHO fraction of the halo.