MICROLENSING TOWARDS THE SMALL-MAGELLANIC-CLOUD EROS-2 FIRST YEAR SURVEY

We present here an analysis of the light curves of 5.3 million stars i n the Small Magellanic Cloud observed by EROS (Experience de Recherche d'Objets Sombres). One star exhibits a variation that is best interpr eted as due to gravitational microlensing by an unseen object. This ca ndidate was also reported by the MACHO collaboration. Once corrected f or blending, the Einstein radius crossing time is 123 days, correspond ing to lensing by a Halo object of 2.6(-2.3)(+8.2) M.. The maximum mag nification is a factor of 2.6. The light curve also displays a periodi c modulation with a 2.5% amplitude and a period of 5.1 days. Parallax analysis of the candidate indicates that a Halo lens would need to hav e a mass of at least 0.3 M., although a lens in the SMC could have a m ass as low as 0.07 M.. We esti mate the optical depth for microlensing towards the SMC due to this event to be similar to 3.3 x 10(-7), with an uncertainty dominated by Poisson statistics. We show that this opt ical depth corresponds to about half that expected for a spherical iso thermal Galactic Halo comprised solely of such objects, and that it is consistent with SMC self-lensing if the SMC is elongated along the li ne-of-sight by at least 5 kpc.