A CHARACTERISTIC PLANETARY FEATURE IN DOUBLE-PEAKED, HIGH-MAGNIFICATION MICROLENSING EVENTS

A significant fraction of microlensing planets have been discovered in high-magnification events, and a significant fraction of these events exhibit a double-peak structure at their peak. However, very wide or very close binaries can also produce double-peaked high-magnification events, with the same gross properties as those produced by planets. Traditionally, distinguishing between these two interpretations has relied on detailed modeling, which is both time-consuming and generally does not provide insight into the observable properties that allow discrimination between these two classes of models. We study the morphologies of these two classes of double-peaked, high-magnification events and identify a simple diagnostic that can be used to immediately distinguish between perturbations caused by planetary and binary companions, without detailed modeling. This diagnostic is based on the difference in the shape of the interpeak region of the light curves. The shape is smooth and concave for binary lensing, while it tends to be either boxy or convex for planetary lensing. In planetary lensing this interpeak morphology is due to the small, weak cusp of the planetary central caustic located between the two stronger cusps. We apply this diagnostic to five observed double-peaked, high-magnification events to infer their underlying nature. A corollary of our study is that good coverage of the interpeak region of double-peaked, high-magnification events is likely to be important for their unique interpretation.