Interpretation of gravitational microlensing by binary systems

Abstract

The fraction of massive compact halo objects (MACHOs) that may be binary systems is unknown, but there is no a priori reason to assume that it is small. If stellar systems can provide useful guidelines, then the fraction may be close to one-half. It is therefore possible that some of the microlensing events, including both those due to stellar systems and those due to MACHOs, may exhibit the complex light curves associated with binarity. In this work we propose a method to search for evidence of binarity in the microlensing light curves and demostrate that our approach can be used to extract binary parameters directly from the light curves. The backdrop of our method is the establishment of a library of binary events where the most important features (including the positions, amplitudes, and measures of asymmetry) are stored. This library allows us to identify a relatively small number of candidate events that mimic the main features of any given light curve. These candidate events provide the input for a chi-squared minimization routine which identifies the parameters associated with the best fit. We find that the solutions can be degenerate, with the degeneracy being more pronounced for weak and moderate events. While a densely sampled light curve and good photometry can better constrain the binary parameters, we find that samplings that are well within the reach of the current generation of observational programs should be sufficient to shed light on the issue of what fraction of lensing objects are in binaries and on some of the properties of the binaries themselves.