Microlensing by a prolate all-macho halo

It is widely believed that dark matter halos are flattened, that is, closer to oblate than prolate. The evidence cited is based largely on observations of galaxies that do not look anything like our own and on numerical simulations that use ad hoc initial conditions. Given what we believe to be a "reasonable doubt" concerning the shape of the dark Galactic halo, we calculate the optical depth and event rate for microlensing of stars in the Large Magellanic Cloud assuming a wide range of models that include both prolate and oblate halos. We find, in agreement with previous analyses, that the optical depth for a spherical (EO) halo and for an oblate (E6) halo are roughly the same, essentially because two competing effects cancel approximately. However, the optical depth for an E6 prolate halo is reduced by ~35%. This means that an all-MACHO prolate halo with reasonable parameters for the Galaxy is consistent with the published microlensing event rate.