STScI Preprint #1292

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, if it is a normal star, 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 suggests 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 for which we have independent information on the location of the lens is the binary lens event MACHO LMC-9, where the caustic crossing time scale suggests the lens to be within the LMC. Thus, our current knowledge of the events indicates that all the three 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.

Status:

Appeared in: The Astrophysical Journal, 508:L147-L150, 1998 December 1

Affiliations:

¹) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
²) NASA/Goddard Space Flight Center, Code 681, Greenbelt, MD 20771
³) National Optical Astronomy Observatories, 950 N. Cherry Avenue, Tucson, AZ 87519-4933