2006ApJ...645..835L

The magnification induced by gravitational microlensing is sensitive to the size of a source relative to the Einstein radius, the natural microlensing scale length. This paper investigates the effect of source size in the case in which the microlensing masses are distributed with a bimodal mass function, with solar-mass stars representing the normal stellar masses and smaller masses (down to 8.5x10^–5 M_☉) representing a dark matter component. It is found that there exists a critical regime in which the dark matter is initially seen as individual compact masses, but with increasing source size the compact dark matter acts as a smooth mass component. This study reveals that interpretation of microlensing light curves, especially claims of small-mass dark matter lenses embedded in an overall stellar population, must consider the important influence of the size of the source.