A Short, Nonplanetary, Microlensing Anomaly: Observations and Light‐Curve Analysis of MACHO 99‐BLG‐47

We analyze PLANET and MACHO observations of MACHO 99-BLG-47, the firstnearly-normal microlensing event for which high signal-to-noise-ratio datareveal a well-covered, short-duration anomaly. This anomaly occurs near the thepeak of the event. Short-duration anomalies near the peak of otherwise normalevents are expected to arise both from extreme-separation (either very close orvery wide), roughly equal-mass binary lenses, and from planetary systems. Weshow that the lens of MACHO 99-BLG-47 is in fact an extreme-separation binary,not a planetary system, thus demonstrating for the first time that these twoimportant classes of events can be distinguished in practice. However, we findthat the wide-binary and close-binary lens solutions fit the data equally well,and cannot be distinguished even at Delta-chi^2=1. This degeneracy isqualitatively much more severe than the one identified for MACHO 98-SMC-1because the present degeneracy spans two rather than one dimension in themagnification field and does not require significantly different blendingfractions. In the appendix, we explore this result, and show that it is relatedto the symmetry in the lens equation.