A Short-Timescale Candidate Microlensing Event in the POINT-AGAPE Pixel Lensing Survey of M31

We report the discovery of a short-duration microlensing candidate in the northern field of the POINT-AGAPE pixel lensing survey toward M31. Almost certainly, the source star has been identified on Hubble Space Telescope archival images, allowing us to infer an Einstein crossing time of tE = 10.4 days, a maximum magnification of Amax ~ 18, and a lens-source proper motion μrel > 0.3 μas day-1. The event has a projected separation of 8' from the center of M31, beyond the bulk of the stellar lens population. There are three plausible identifications/locations for the lensing object: a massive compact halo object (MACHO) in either M31 or the Milky Way, or a star in the M31 disk. The most probable mass is 0.06 M☉ for an M31 MACHO, 0.02 M☉ for a Milky Way MACHO, and 0.2 M☉ for an M31 stellar lens. While the stellar interpretation is possible, the MACHO interpretation is the most probable for halo fractions above 20%.