Bulge microlensing optical depth from EROS 2 observations

We present a measurement of the microlensing optical depth toward the Galactic bulge based on the analysis of 15 contiguous 1 deg² fields centered on (l=2.5^o, b=-4.0^o) and containing N_*=1.42x 10⁶ clump-giant stars (belonging to the extended clump area) monitored during almost three bulge seasons by EROS (Expérience de Recherche d'Objets Sombres). We find tau_bulge =0.94+/- 0.29x 10^-6 averaged over all fields, based on 16 microlensing events with clump giants as sources. This value is substantially below several other determinations by the MACHO and OGLE groups and is more in agreement with what is expected from axisymmetric and non-axisymmetric bulge models.

Based on observations made with the MARLY telescope at the European Southern Observatory, La Silla, Chile.