Distinguishing between binary-source and planetary microlensing perturbations

A planetary microlensing event is characterized by a short-lived perturbation to the standard microlensing curve. Planetary perturbations typically last from a few hours to a day,have maximum amplitudes, delta(max), of +/-5%-20% of the standard curve, and come in two classes: major and minor image perturbations. There exist a subset of binary-source events that can reproduce the main features of major image perturbations, which are likely to represent more than half of all planetary events, and thus masquerade as planetary events. These events require a binary source with a small flux ratio, epsilon similar to 10(-2) to 10(-4), and a small impact parameter for the fainter source, beta(2) less than or similar to epsilon/delta(max). The detection probability of events of this type is similar to beta(2), and can be as high as similar to 30%; this is comparable to planetary detection rates. Thus a sample of planetary-like perturbations could be seriously contaminated by binary-source events, and there exists the possibility that completely meaningless physical parameters would be derived for any given major image perturbation. Here I derive analytic expressions for a binary-source event in the extreme flux-ratio limit and use these to demonstrate the basic degeneracy between binary-source and planet perturbations. I describe how the degeneracy can be broken by dense and accurate sampling of the perturbation, optical/infrared photometry, or spectroscopic measurements.