The Many Possible Interpretations of Microlensing Event OGLE 2002‐BLG‐055

Abstract

Microlensing event OGLE 2002-BLG-055 is characterized by a smooth, slightly asymmetric single-lens curve with an isolated, secure data point that is ~0.6 mag brighter than neighboring points separated by a few days. It was previously suggested that the single deviant data point and global asymmetry were best explained by a planetary companion to the primary lens with mass ratio q = 10-3 to 10-2 and by parallax effects induced by the motion of the Earth. We revisit the interpretation of OGLE 2002-BLG-055 and show that the data can be explained by a wide variety of models. We find that the deviant data point can be fitted by a large number of qualitatively different binary-lens models whose mass ratios range, at the ~3 σ level, from q 10-4 to 10-1. This range is consistent with a planet, brown dwarf, or M dwarf companion for reasonable primary masses of M 0.8 M☉. A subset of these binary-lens fits consists of a family of continuously degenerate models whose mass ratios differ by an order of magnitude, but whose light curves differ by 2% for the majority of the perturbation. The deviant data point can also be explained by a binary companion to the source, with a secondary/primary flux ratio of ~1%. This model has the added appeal that the global asymmetry is naturally explained by the acceleration of the primary induced by the secondary. The binary-source model yields a measurement of the Einstein ring radius projected onto the source plane of E = 1.87 ± 0.40 AU. OGLE 2002-BLG-055 is an extreme example that illustrates the difficulties and degeneracies inherent in the interpretation of weakly perturbed and/or poorly sampled microlensing light curves.