Event Rates and Timescale Distributions from Realistic Microlensing Models of M31

Abstract

We provide a set of microlensing event rate maps for M31, the Andromeda galaxy. Rates for M31 microlensing were calculated on the basis of a four component model of the lens and source populations: disk and bulge sources lensed by the bulge, the disk, the M31 halo, and the Galactic halo lenses. We confirm the high rate gradient along the minor axis of M31 due to a dark halo of lenses. Furthermore, we compute the timescale distributions of events, for both Einstein times and FWHM times. We explore how the rate contours and timescale distributions can be used to measure the shape and extent of the microlensing halo. With one year of twice-weekly sampling, or three observing seasons, a halo MACHO fraction as small as 5% can be detected with modest ground-based telescopes.