A binary lensing event toward the LMC: Observations and dark matter implications

The MACHO collaboration has recently analyzed 2.1 years of photometric data for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis has revealed 8 candidate microlensing events and a total microlensing optical depth of tau(meas) = 2.9(-0.9)(+1.4) x 10(-7). This significantly exceeds the number of events (1.1) and the microlensing optical depth predicted from known stellar populations: tau(back) = 5.1 x 10(-8), but it is consistent with models in which about half of the standard dark halo mass is composed of Machos of mass similar to 0.5M(.). One of these 8 events appears to be a binary lensing event with a caustic crossing that is partially resolved, and the measured caustic crossing time allows us to estimate the distance to the lenses. Under the assumption that the source star is a single star and not a short period binary, we show that the lensing objects are very likely to reside in the LMC. However, if we assume that the optical depth for LMC-LMC lensing is large enough to account for our entire lensing signal, then the binary event does not appear to be consistent with lensing of a single LMC source star by a binary residing in the LMC. Thus, while the binary lens may indeed reside in the LMC, there is no indication that most of the lenses reside in the LMC.