Galactic Microlensing as a Method of Detecting Massive Compact Halo Objects

The dark matter of our Galaxy may well consist of Jupiters, brown dwarfs, or the remnants of an early generation of stars. In 1986, Paczynski suggested that a population of such objects could be detected by watching for microlensing of stars in the LMC. Since several groups are now preparing to undertake such a search, we reconsider in detail Paczynski's idea. Using a more realistic model of the halo density and velocity structure, we recalculate the microlensing optical depth, the microlensing event rate and the average duration of an event, correcting an error, but finding rough agreement with Paczynski's estimates. We also calculate the distribution of microlensing events as a function of their duration and amplitude, finding that photometric measurements more frequent than the average event duration are needed to detect a substantial fraction of the events. We also find an enhancement in the microlensing rate due to the motion of the Sun and LMC, and a seasonal modulation of the rate due to the motion of the Earth. Finally, we discuss what limits could be placed on the halo density from the successful completion of such an observational program, and mention some possible problems.