Magnitude bias of microlensed sources towards the Large Magellanic Cloud

There are lines of evidence suggesting that some of the observed microlensing events in the direction of the Large Magellanic Cloud (LMC) are caused by ordinary star lenses as opposed to dark Machos in the Galactic halo. Efficient lensing by ordinary stars generally requires the presence of one or more additional concentrations of stars along the line of sight to the LMC disk. If such a population behind the LMC disk exists, then the source stars (for lensing by LMC disk objects) will be drawn preferentially from the background population and will show systematic differences from LMC field stars. One such difference is that the (lensed) source stars will be farther away than the average LMC field stars, and this should be reflected in their apparent baseline magnitudes. We focus on red clump stars: these should appear in the color-magnitude diagram at a few tenths of a magnitude fainter than the field red clump. Suggestively, the only near-clump confirmed event, MACHO-LMC-1, is a few tenths of magnitude fainter than the clump. Monte Carlo tests suggest that only one additional clump event could rule out the Galactic halo lensing hypothesis at the 95% confidence level, although more events may be needed if some of the observed events are due to the Galactic halo (Macho) lenses. It will be interesting to apply the method in a few years with a bigger sample of clump events. Subject headings: Magellanic Clouds — Galaxy: structure