EDP Sciences
Free access
Volume 445, Number 2, January II 2006
Page(s) 375 - 385
Section Astrophysical processes
DOI http://dx.doi.org/10.1051/0004-6361:20053502

A&A 445, 375-385 (2006)
DOI: 10.1051/0004-6361:20053502

Monte Carlo analysis of MEGA microlensing events towards M 31

G. Ingrosso1, S. Calchi Novati2, F. De Paolis1, Ph. Jetzer2, A. A. Nucita1 and F. Strafella1

1  Dipartimento di Fisica, Università di Lecce and INFN, Sezione di Lecce, CP 193, 73100 Lecce, Italy
2  Institute for Theoretical Physics, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

(Received 24 May 2005 / Accepted 6 September 2005)

We perform an analytical study and a Monte Carlo (MC) analysis of the main features for microlensing events in pixel lensing observations towards M 31. 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 M 31 and the standard model for our galaxy, we estimate the MACHO-to-self lensing probability and the event time duration towards M 31. 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 t1/2 and the magnitude at maximum  $R_{\rm max}$. For a MACHO mass of  $0.5~M_{\odot}$ we find typical values of $t_{1/2} \simeq 20$ day and $R_{\rm {max}} \simeq 22$, for both MACHO-lensing and self-lensing events occurring beyond about 10 arcmin from the M 31 center. A comparison of the observed features (t1/2 and $R_{\rm max}$) 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.

Key words: gravitational lensing -- galaxy: halo -- galaxies: individuals: M 31

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