Spectroscopy of MACHO 97-SMC-1: self-lensing within the SMC

More than a dozen microlensing events have been detected so far towards the LMC and 2 towards the SMC. If all the lenses are in the Galactic halo, both the LMC and the SMC events are expected to have similar time scales. However, the first event towards the SMC, MACHO 97-SMC-1, had a time scale of 123 days which is much larger than the typical time scale for the LMC events. Since the observed time scale of the SMC event would need the mass of the halo lens to be ∼3 M⊙, it has been argued earlier that the lens must be within the SMC, which we spectroscopically confirm in this paper. From optical depth estimates, we first show that the stars within the SMC play a dominant role as gravitational lenses and can fully account for the observed microlensing events, mainly due to its large physical depth. We also show that if the lenses are within the Magellanic Clouds, then the SMC events should be longer in duration than the LMC events, a fact that is consistent with the observations. The time scale of the event implies that the mass of the lens is ∼ 2 M⊙ if it is in the Milky Way disk or halo, in which case the lens is expected to be bright and should reveal itself in the spectrum. Here, we present an optical spectrum of MACHO 97-SMC-1 obtained in May 1997 which shows that the source is a main-sequence B star. There is no trace of any contribution from the lens which demonstrates that the lens is not in the Milky Way disk or halo, but is a low-mass star within the SMC. It is worth noting here that MACHO SMC-98-1 is the only other observed event towards the SMC. This was a binary lens event where the caustic crossing time-scale as observed by PLANET, MACHO, EROS and OGLE collaborations, suggests that the lens is within the SMC. Furthermore, the only LMC event where the location of the lens is known is the binary lens event MACHO LMC-9, for which the lens is also within the LMC. Thus, all the 3 microlensing events towards the Magellanic Clouds for which we have independent knowledge of the location of the lenses are due to self-lensing within the Magellanic Clouds.