A Candidate M31/M32 Intergalactic Microlensing Event

We report the discovery of a microlensing candidate projected 2'54'' from the center of M32, on the side closest to M31. The blue color (R-I = 0.00 ± 0.14) of the source argues strongly that it lies in the disk of M31, while the proximity of the line of sight to M32 implies that this galaxy is the most likely host of the lens. If this interpretation is correct, it would confirm previous arguments that M32 lies in front of M31. If more events are discovered in this direction in a dedicated experiment, they could be used to measure the mass function of M32 up to an unknown scale factor. By combining microlensing observations of a binary-lens event with a measurement of the M31-M32 relative proper motion using the astrometric satellites Space Interferometry Mission or Global Astrometric Interferometer for Astrophysics, it will be possible to measure the physical separation of M31 and M32, the last of the six phase-space coordinates needed to assign M32 an orbit.