Discovery and characterization of a caustic crossing microlensing event inthe 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 caust ic crossing binary microlensing event toward the Magellanic Clouds to be di scovered in progress. Frequent coverage of the evolving light curve allowed an accurate predictio n for the date of the source crossing out of the lens caustic structure. Th e caustic crossing temporal width, along with the angular size of the sourc e star, measures the proper motion of the lens with respect to the source a nd thus allows an estimate of the location of the lens. Lenses located in t he Galactic halo would have a velocity projected to the SMC of <(upsilon)ov er cap> similar to 1500 kms(-1), while an SMC lens would typically have <(u psilon)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 th e proper motion of the lensing system. We have performed a joint fit to the MACHO/GMAN data presented here, includ ing recent EROS data of this event from Afonso and collaborators. These joi nt data are sufficient to constrain the time t(*) for the lens to move an a ngle equal to the source angular radius: t(*) = 0.116 +/- 0.010 days. We es timate a radius for the lensed source of R-* = 1.1 +/- 0.1 R. from its unbl ended color and magnitude. This yields a projected velocity of <(upsilon)ov er 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 S MC lenses would be expected to have <(upsilon)over cap> as large as this. T his implies that the lensing system is more likely to reside in the SMC tha n in the Galactic halo. Similar observations of future Magellanic Cloud mic rolensing events will help to determine the contribution of MACHOS to the G alaxy's dark halo.