Microlensing of Blended Stellar Images

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Abstract
A popular model of single microlensing light curves neglects the possibility that only a fraction of the light is due to the lensed star, the remaining part being due to a close, unresolved blend, which may be related or unrelated to the lens. Unfortunately, the effects of blending are significant, as all microlensing experiments choose very crowded fields as their targets. The propensity of blends among double lenses is the most direct evidence for the seriousness of the problem. In this paper, we point out a strong degeneracy of the fitting procedure for single lensing events, which makes it very difficult to detect the presence of a blend, and practically impossible to correct for it by purely photometric means, in a large part of the parameter space. Some blends may be detected by astrometric means, but the majority have to be corrected for statistically. It will be helpful to measure the luminosity function well below the ground-based detection limit, e.g., using the Hubble Space Telescope (HST). Binary stars in the target population affect blending and may be undetectable with HST; their presence will be revealed by repeating microlensing events, so it will be possible to correct for them statistically. If no correction is made, then the event timescales and the lens masses are systematically underestimated.
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