Discovery and characterization of a caustic crossing microlensing event in the Small Magellanic Cloud

We present photometric observations and analysis of the second microlensing event detected toward the Small Magellanic Cloud (SMC), MACHO Alert 98-SMC-1. This event was detected early enough to allow intensive observation of the light curve. These observations revealed 98-SMC-1 to be the first caustic crossing binary microlensing event toward the Magellanic Clouds to be discovered in progress. Frequent coverage of the evolving light curve 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 <(upsilon)over cap> similar to 1500 kms(-1), while an SMC lens would typically have <(upsilon)over cap> similar to 60 kms(-1). The event light curve 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 from Afonso and collaborators. 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 R-* = 1.1 +/- 0.1 R. from its unblended color and magnitude. This yields a projected velocity of <(upsilon)over cap> = 76 +/- 10 kms(-1). Only 0.12% of halo lenses would be expected to have a <(upsilon)over cap> value at least as small as this, while 38% of SMC lenses would be expected to have <(upsilon)over cap> as large as this. This implies that the lensing system is more likely to reside in the SMC than in the Galactic halo. Similar observations of future Magellanic Cloud microlensing events will help to determine the contribution of MACHOS to the Galaxy's dark halo.