Microlensing events of the LMC are better explained by stars within the LMC than by MACHOs

Sahu, Kailash C.
Bibliographical reference

Astronomical Society of the Pacific, Publications (ISSN 0004-6280), vol. 106, no. 703, p. 942-948

Advertised on:
9
1994
Description
The recently reported microlensing events of the Large Magellanic Cloud (LMC) have caused much excitement, and have been interpreted as due to 'dark objects' called massively compact halo objects (MACHOs) in the halo of our Galaxy. It is shown here that stars within the LMC play a dominant role as gravitational lenses and can indeed account for the observed events. For observations within the bar of the LMC, the probability of microlensing being caused by a star within the LMC is found to be approximately 5 x 10-8. Outside of the bar, the probability of microlensing being caused by a star in the LMC is 4-12 times lower. The MACHO event (Alcock et al. 1993) and one of the Earth Resources Observation Satellites (EROS) events (Aubourg et al. 1993) lie within the bar for which the probability of microlensing is consistent with being caused by an object within the LMC. If stars within the LMC play a dominant role as lenses, the events should be concentrated towards the center of the LMC. On the other hand, if MACHOs play a dominant role as lenses then, for a given number of monitored stars, the events should be uniformly distributed over the whole area of the LMC. Thus the galactic and the LMC lenses can be statistically distinguished in most cases. It is further shown that, under certain conditions, the light curve of an event caused by a star within the LMC would be different from the one caused by a MACHO. This can also be a distinguishing signature, and seems to have been observed in case of the MACHO event. The fit discrepancy near the peak which the authors say 'is not yet understood' is reduced if the event is caused by an object within the LMC, which further suggests that the lensing is due to a low-mass star within the LMC itself.