We report the discovery of a short-duration microlensing candidate in the northern field of the POINT-AGAPE pixel lensing survey towards M31 .
The full-width half-maximum timescale is very short , t _ { 1 / 2 } = 1.8 days .
Almost certainly , the source star has been identified on Hubble Space Telescope archival images , allowing us to infer an Einstein crossing time of t _ { E } = 10.4 days , a maximum magnification of A _ { max } \sim 18 , and a lens-source proper motion \mu _ { rel } > 0.3 \mu as/day .
The event lies projected at 8 ^ { \prime } from the center of M31 , which is beyond the bulk of the stellar lens population .
The lens is likely to reside in one of three locations .
It may be a star in the M31 disk , or a massive compact halo object ( Macho ) in either M31 or the Milky Way .
The most probable mass is 0.06 { M } _ { \odot } for an M31 Macho , 0.02 { M } _ { \odot } for a Milky Way Macho and 0.2 { M } _ { \odot } for an M31 stellar lens .
Whilst the stellar interpretation is plausible , the Macho interpretation is the most probable for halo fractions above 20 \% .