MICROLENSING BY A PROLATE ALL-MACHO HALO

It is widely believed that dark matter halos are flattened, that is, c loser to oblate than prolate. The evidence cited is based largely on o bservations of galaxies that do not look anything like our own and on numerical simulations that use ad hoc initial conditions. Given what w e believe to be a ''reasonable doubt'' concerning the shape of the dar k Galactic halo, we calculate the optical depth and event rate for mic rolensing of stars in the Large Magellanic Cloud assuming a wide range of models that include both prolate and oblate halos. We find, in agr eement with previous analyses, that the optical depth for a spherical (EO) halo and for an oblate (E6) halo are roughly the same, essentiall y because two competing effects cancel approximately. However, the opt ical depth for an E6 prolate halo is reduced by similar to 35%. This m eans that an all-MACHO prolate halo with reasonable parameters for the Galaxy is consistent with the published microlensing event rate.