The MACHO Project First Year LMC Results: The Microlensing Rate and the Nature of the Galactic Dark Halo

The MACHO collaboration reports on the analysis of our first year LMC data, 9.5 million light curves with an average of 235 observations each. Automated selection procedures give 3 events consistent with microlensing. We evaluate our experimental detection efficiency using a range of Monte- Carlo simulations. Using a `standard' halo density profile we find that a halo comprised entirely of Machos in the mass range 3 \ten{-4} to 0.06 \msun would predict > 15 detected events in this dataset; thus a standard spherical halo cannot be dominated by objects in this mass range. Assuming all three events are microlensing of halo objects and fitting a naive spherical halo model to our data yields a Macho halo fraction f =0.19+0.16-0.10, a total mass in Machos (inside 50 kpc) of 7.6+6-4 \ten{10} \msun, and a microlensing optical depth 8.8+7-5 \ten{-8} (68\% CL). Exploring a wide range of halo models we find that our constraints on the Macho fraction are quite model-dependent, but constraints on the total mass in Machos within 50 kpc are quite secure.