IDENTIFYING MICROLENSING BY BINARIES

The microlensing monitoring programs have studied large numbers of sta ndard light curves that seem to be due to lensing by a dark point mass . Theory predicts that many microlensing events should display signifi cant deviations from the standard form. Lens binarity, in particular, is expected to be common. So far, however, only a handful of light cur ves exhibit evidence that the lens is a binary; all of these display d ramatic deviations from the standard light curve, exhibiting pronounce d multiple peaks, caustic crossings, or both. Binary lens events in wh ich the light curve is less dramatically perturbed should also exist i n the data set. Why, then, have we not detected them? The answer may l ie in the fact that the perturbations, though often significant, tend to be less distinctive than those associated with caustic crossings. I t is therefore possible that some of these more gently perturbed event s have been misclassified, and that others have simply been missed. Re liable estimates of the overall detection efficiency, and hence deriva tion of the fraction of the Galactic halo mass that may be in MACHOs, rely on resolving this issue. Microlensing can also be used to determi ne the form of the initial mass function (IMF) beyond the solar neighb orhood; accurate determination of the IMF also relies on the ability t o correctly identify binary light curves. We present a method to deter mine whether a light curve is due to lensing by a binary. The method w orks for both gently and dramatically perturbed binary-lens light curv es. Our method identifies all degenerate solutions-i.e., all possible lensing events that might have given rise to the observed light curve. It also enables us to eliminate from consideration large ranges of po ssible false positive identifications associated with light curves tha t might mimic microlensing by a binary. This method, or a generalizati on of it, can also be applied to the analysis of light curves that dev iate from the standard point-mass lens form because of astronomical ef fects other than lens binarity.