The MACHO project: Microlensing results from 5.7 years of Large MagellanicCloud observations

We report on our search for microlensing toward the Large Magellanic Cloud (LMC). Analysis of 5.7 yr of photometry on 11.9 million stars in the LMC re veals 13-17 microlensing events. A detailed treatment of our detection effi ciency shows that this is significantly more than the similar to2-4 events expected from lensing by known stellar populations. The timescales ((t) ove r cap) of the events range from 34 to 230 days. We estimate the microlensin g optical depth toward the LMC from events with 2 < <(t)over cap> < 400 day s to be <tau>(400)(2) = 1.2(-0.3)(+0.4) x 10(-7), with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inco nsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the cont ext of a Galactic dark matter halo, consisting partially of compact objects , a maximum-likelihood analysis gives a MACHO halo fraction of 20% for a ty pical halo model with a 95% confidence interval of 8%-50%. A 100% MACHO hal o is ruled out at the 95% confidence level for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACH O mass is between 0.15 and 0.9 Mo, depending on the halo model, and the tot al mass in MACHOs out to 50 kpc is found to be 9(-3)(+4) x 10(10) M., indep endent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of 2. This is mostly due to Poisson noise, because with 3.4 times more exposure and increased sensit ivity to long-timescale events, we did not find the expected factor of simi lar to4 more events. In addition to a larger data set, this work also inclu des an improved efficiency determination, improved likelihood analysis, and more thorough testing of systematic errors, especially with respect to the treatment of potential backgrounds to microlensing. We note that an import ant source of background are supernovae (SNe) in galaxies behind the LMC.