Microlens Parallaxes of Binary Lenses Measured from a Satellite

Abstract

Caustic-crossing binary lenses make up about 5% of all detected microlenses. The relative proper motion of a caustic-crossing binary lens can be measured with observations from a single terrestrial telescope. Thus, uniquely, binary lenses can be completely solved with only the addition of a measurement of the microlensing parallax. This solution will yield the mass, distance, and transverse velocity of the lens relative to the source. To date, only one of the $\sim 1000$ observed microlensing events has been so solved. We examine the ability of a parallax satellite combined with ground-based observations to solve these events. To measure both components of the vector parallax, the lens must be observed near two different caustics. Thus, the final accuracy is determined mostly by whether one can intensively monitor part of the first caustic crossing, by the magnification pattern, and by the path of the source with respect to the lens geometry. We find that vector parallaxes can be measured far more easily for binary lenses than single lenses, requiring 1-3 orders of magnitude fewer photons. They may thus yield a large number of completely solved lenses relatively cheaply.