The Anomaly in the Candidate Microlensing Event PA-99-N2

The light curve of PA-99-N2, one of the recently announced microlensing candidates toward M31, shows small deviations from the standard Paczyński form. We explore a number of possible explanations, including correlations with the seeing, the parallax effect, and a binary lens. We find that the observations are consistent with an unresolved red giant branch or asymptotic giant branch star in M31 being microlensed by a binary lens. We find that the best-fit binary lens mass ratio is ~1.2 × 10^-2, which is one of the most extreme values found for a binary lens so far. If both the source and lens lie in the M31 disk, then the standard M31 model predicts the probable mass range of the system to be 0.02-3.6 _☉ (95% confidence limit). In this scenario, the mass of the secondary component is therefore likely to be below the hydrogen-burning limit. On the other hand, if a compact halo object in M31 is lensing a disk or spheroid source, then the total lens mass is likely to lie between 0.09 and 32 _☉, which is consistent with the primary being a stellar remnant and the secondary being a low-mass star or brown dwarf. The optical depth (or, alternatively, the differential rate) along the line of sight toward the event indicates that a halo lens is more likely than a stellar lens, provided that dark compact objects comprise no less than 15% (or 5%) of halos.