  We perform an analytical study and a Monte Carlo (MC) analysis of the main
features for microlensing events in pixel lensing observations towards M31. Our
main aim is to investigate the lens nature and location of the 14 candidate
events found by the MEGA collaboration. Assuming a reference model for the mass
distribution in M31 and the standard model for our galaxy, we estimate the
MACHO-to-self lensing probability and the event time duration towards M31.
Reproducing the MEGA observing conditions, as a result we get the MC event
number density distribution as a function of the event full-width half-maximum
duration $t_{1/2}$ and the magnitude at maximum $R_{\mathrm {max}}$. For a
MACHO mass of $0.5 M_{\odot}$ we find typical values of $t_{1/2} \simeq 20$ day
and $R_{\mathrm {max}} \simeq 22$, for both MACHO-lensing and self-lensing
events occurring beyond about 10 arcminutes from the M31 center. A comparison
of the observed features ($t_{1/2}$ and $R_{\mathrm {max}}$) with our MC
results shows that for a MACHO mass $>0.1 M_{\odot}$ the four innermost MEGA
events are most likely self-lensing events, whereas the six outermost events
must be genuine MACHO-lensing events.
