A Complete Set of Solutions For Caustic-Crossing Binary Microlensing Events

We present a method to analyze binary-lens microlensing light curves with one well-sampled fold caustic crossing. In general, the surface of χ shows extremely complicated behavior over the 9-parameter space that characterizes binary lenses. This makes it difficult to systematically search the space and verify that a given local minimum is a global minimum. We show that for events with well-monitored caustics, the caustic-crossing region can be isolated from the rest of the light curve and easily fit to a 5-parameter function. Four of these caustic-crossing parameters can then be used to constrain the search in the larger 9-parameter space. This allows a systematic search for all solutions and thus identification of all local minima. We illustrate this technique using the PLANET data for MACHO 98-SMC-1, an excellent and publicly available caustic-crossing data set. We show that a very broad range Univ. of Canterbury, Dept. of Physics & Astronomy, Private Bag 4800, Christchurch, New Zealand Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands South African Astronomical Observatory, P.O. Box 9, Observatory 7935, South Africa Ohio State University, Department of Astronomy, Columbus, OH 43210, U.S.A. Univ. of Tasmania, Physics Dept., G.P.O. 252C, Hobart, Tasmania 7001, Australia Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD. 21218 U.S.A. Perth Observatory, Walnut Road, Bickley, Perth 6076, Australia