AgapeZ1: a large amplification microlensing event or an odd variable star towards the inner bulge of M31

AgapeZ1 is the brightest and the shortest duration microlensing candidate event found to date in the Agape experiment. It occurred only 4200 from the centre of M31 at RA = 0h42m41:47s and Dec = 41°16'39.1" (J2000). Our photometry shows that the half intensity duration of the event is 5.3 days and at maximum brightness its magnitude is R = 17:9 (MR ~ −6) with B − R = 0:80 mag colour. A search on HST archives produced a single resolved star within the projected event position error box. Its magnitude is R = 22, and its colour is compatible with that of the event at the 2σ level. If this event is identified with the HST star, the implied amplification is about 4 magnitudes or 40 in brightness. This would lead to an Einstein crossing time radius of about 60 days. AgapeZ1 could be a bulge/bulge microlensing event involving a binary star.