Identifying microlensing by binaries

The microlensing monitoring programs have studied large numbers of standard light curves that seem to be due to lensing by a dark point mass. Theory predicts that many microlensing events should display significant deviations from the standard form. Lens binarity, in particular, is expected to be common. So far, however, only a handful of light curves exhibit evidence that the lens is a binary; all of these display dramatic deviations from the standard light curve, exhibiting pronounced multiple peaks, caustic crossings, or both. Binary lens events in which the light curve is less dramatically perturbed should also exist in the data set. Why, then, have we not detected them? The answer may lie in the fact that the perturbations, though often significant, tend to be less distinctive than those associated with caustic crossings. It is therefore possible that some of these more gently perturbed events have been misclassified, and that others have simply been missed. Reliable estimates of the overall detection efficiency, and hence derivation of the fraction of the Galactic halo mass that may be in MACHOs, rely on resolving this issue. Microlensing can also be used to determine the form of the initial mass function (IMF) beyond the solar neighborhood; accurate determination of the IMF also relies on the ability to correctly identify binary light curves. We present a method to determine whether a light curve is due to lensing by a binary. The method works for both gently and dramatically perturbed binary-lens light curves. 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 possible false positive identifications associated with light curves that might mimic microlensing by a binary. This method, or a generalization of it, can also be applied to the analysis of light curves that deviate from the standard point-mass lens form because of astronomical effects other than lens binarity.