The Anomaly in the Candidate Microlensing Event PA99N2
Abstract
The light curve of PA99N2, 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 bestfit 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 0.023.6 M_{solar} (95% confidence limit). In this scenario, the mass of the secondary component is therefore likely to be below the hydrogenburning 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 M_{solar}, which is consistent with the primary being a stellar remnant and the secondary being a lowmass 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.
 Publication:

The Astrophysical Journal
 Pub Date:
 February 2004
 DOI:
 10.1086/380820
 arXiv:
 arXiv:astroph/0310457
 Bibcode:
 2004ApJ...601..845A
 Keywords:

 Cosmology: Dark Matter;
 Galaxies: Individual: Messier Number: M31;
 Cosmology: Gravitational Lensing;
 Astrophysics
 EPrint:
 Latex, 23 pages, 9 figures, in press at The Astrophysical Journal