Discovery and Characterization of a Caustic Crossing Microlensing Event in the SMC

We present photometric observations and analysis of the second microlensing event detected towards the Small Magellanic Cloud (SMC), MACHO Alert 98-SMC-1. This event was detected early enough to allow intensive observation of the lightcurve. These observations revealed 98-SMC-1 to be the first caustic crossing, binary microlensing event towards the Magellanic Clouds to be discovered in progress. Frequent coverage of the evolving lightcurve allowed an accurate prediction for the date of the source crossing out of the lens caustic structure. The caustic crossing temporal width, along with the angular size of the source star, measures the proper motion of the lens with respect to the source, and thus allows an estimate of the location of the lens. Lenses located in the Galactic halo would have a velocity projected to the SMC of v̂ ∼ 1500 km s, while an SMC lens would typically have v̂ ∼ 60 km s. The event lightcurve allows us to obtain a unique fit to the parameters of the binary lens, and to estimate the proper motion of the lensing system. We have performed a joint fit to the MACHO/GMAN data presented here, including recent EROS data of this event (Afonso et al. 1998). These joint data are sufficient to constrain the time t∗ for the lens to move an angle equal to the source angular radius; t∗ = 0.116 ± 0.010 days. We estimate a radius for the lensed source of