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 tota l 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 microlen sing 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 t o 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 c austic 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 sig nal, then the binary event does not appear to be consistent with lensi ng 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 indica tion that most of the lenses reside in the LMC.