The M31 microlensing event WeCAPP-GL1/Point-AGAPE-S3: evidence for a MACHO component in the dark halo of M31?

Abstract

We re-analyze the M31 microlensing event WeCAPP-GL1/Point-AGAPE-S3 taking into account that stars are not point-like but extended. We show that the finite size of stars can dramatically change the self-lensing eventrate and (less dramatically) also the halo lensing eventrate, if events are as bright as WeCAPP-GL1. The brightness of the brightest events mostly depends on the source sizes and fluxes and on the distance distribution of sources and lenses and therefore can be used as a sensitive discriminator between halo-lensing and self-lensing events, provided the stellar population mix of source stars is known well enough. Using a realistic model for the 3D-light distribution, stellar population and extinction of M31, we show that an event like WeCAPP-GL1 is very unlikely to be caused by self-lensing. In the entire WeCAPP-field (\(17.2'\times 17.2'\) centered on the bulge) we expect only one self-lensing event every 49 years with the approximate parameters of WeCAPP-GL1 (time-scale 1-3d, \(R\) flux-excess <19.0 mag). If we assume only 20% of the dark halos of M31 and the Milky-Way consist of 1 solar mass MACHOs an event like WeCAPP-GL1 would occur every 10 years. Further more, if one uses position, FWHM time scale, flux excess and color of WeCAPP-GL1, self-lensing is even 13 times less likely than lensing by a MACHO, if MACHOs contribute 20% to the total halo mass and have masses in the range of 0.1 to 4 solar masses. We also demonstrate that (i) the brightness distribution of events in general is a good discriminator between self and halo lensing (ii) the time-scale distribution is a good discriminator if the MACHO mass is larger than 0.5 solar masses. Future surveys of M31 like PAndromeda (Pan-STARRS 1) should be able to provide many more such events within the next 4 years.