M31 Pixel Lensing PLAN Campaign: MACHO Lensing and Self Lensing Signals
  We present the final analysis of the observational campaign carried out by
the PLAN (Pixel Lensing Andromeda) collaboration to detect a dark matter signal
in form of MACHOs through the microlensing effect. The campaign consists of
about 1 month/year observations carried out during 4 years (2007-2010) at the
1.5m Cassini telescope in Loiano ("Astronomical Observatory of BOLOGNA", OAB)
plus 10 days of data taken in 2010 at the 2m Himalayan Chandra Telescope (HCT)
monitoring the central part of M31 (two fields of about 13'x12.6'). We
establish a fully automated pipeline for the search and the characterization of
microlensing flux variations: as a result we detect 3 microlensing candidates.
We evaluate the expected signal through a full Monte Carlo simulation of the
experiment completed by an analysis of the detection efficiency of our
pipeline. We consider both "self lensing" and "MACHO lensing" lens populations,
given by M31 stars and dark matter halo MACHOs, in the M31 and the Milky Way
(MW), respectively. The total number of events is compatible with the expected
self-lensing rate. Specifically, we evaluate an expected signal of about 2
self-lensing events. As for MACHO lensing, for full 0.5 (0.01) solar mass MACHO
halos, our prediction is for about 4 (7) events. The comparatively small number
of expected MACHO versus self lensing events, together with the small number
statistics at disposal, do not enable us to put strong constraints on that
population. Rather, the hypothesis, suggested by a previous analysis, on the
MACHO nature of OAB-07-N2, one of the microlensing candidates, translates into
a sizeable lower limit for the halo mass fraction in form of the would be MACHO
population, f, of about 15% for 0.5 solar mass MACHOs.
