A Study into the Feasibility of Obtaining Finite Source Sizes From MACHO-type Microlensing Light Curves

Recent discussion of the effects of finite source size on high magnification microlensing events due to MACHOs motivates a study into the feasibility of observing such effects and extracting the source radius. Simulated observations are generated by adding Gaussian error to points sampled on theoretical microlensing light curves for a limb darkened, extended source. These simulated data sets are fitted in an attempt to see how well the fits extract the radius of the source. The source size can be fitted with reasonable accuracy only if the impact parameter of the event, $p$, is less than the stellar radius, $R_{\star}$. It is possible to distinguish ``crossing'' events, ones where $p<R_{\star}$, from ``non--crossing'' events if the light curve is well sampled around the peak and photometric error is small --- i.e.~$\geq 3$ observations while the lens transits the disk of the source, and $\sigma_{phot}<0.08\;{\rm mag}$. These requirements are just within the reach of current observational programs; the use of an early-warning system and multiple observing sites should increase the likelihood that $R_{\star}$ can be fitted. The programs used to simulate and fit data can be obtained via anonymous {\tt ftp}.