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

Microlensing event OGLE-2002-BLG-055 is characterized by a smooth, slightlyasymmetric single-lens curve with an isolated, secure data point that is ~0.6magnitudes brighter than neighboring points separated by a few days. It waspreviously suggested that the single deviant data point and global asymmetrywere best explained by a planetary companion to the primary lens with massratio log(q)=-3 to -2, and parallax effects induced by the motion of the Earth.We revisit the interpretation of OGLE-2002-BLG-055, and show that the data canbe explained by wide variety of models. We find that the deviant data point canbe fit by a large number of qualitatively different binary-lens models whosemass ratios range, at the ~3-sigma level, from log(q) ~ -4 to -1. This range isconsistent with a planet, brown dwarf, or M-dwarf companion for reasonableprimary masses of M> 0.8 M_sun. A subset of these binary-lens fits consist of afamily of continuously degenerate models whose mass ratios differ by anorder-of-magnitude, but whose light curves differ by <2% for the majority ofthe perturbation. The deviant data point can also explained by a binarycompanion to the source with secondary/primary flux ratio of ~1%. This modelhas the added appeal that the global asymmetry is naturally explained by theacceleration of the primary induced by the secondary. The binary-source modelyields a measurement of the Einstein ring radius projected on source plane of\hat r_E=1.87 +/- 0.40 AU. OGLE-2002-BLG-055 is an extreme example thatillustrates the difficulties and degeneracies inherent in the interpretation ofweakly perturbed and/or poorly sampled microlensing light curves.Comment: 8 pages, 3 figures, 1 table. Minor changes. Accepted to ApJ, to appear in the August 10, 2004 issue (v611