Microlensing by a Prolate All‐MACHO Halo

It is widely believed that dark matter halos are flattened, that is closer tooblate than prolate. The evidence cited is based largely on observations ofgalaxies which do not look anything like our own and on numerical simulationswhich use ad hoc initial conditions. Given what we believe to be a ``reasonabledoubt'' concerning the shape of dark Galactic halo we calculate the opticaldepth and event rate for microlensing of stars in the LMC assuming a wide rangeof models that include both prolate and oblate halos. We find, in agreementwith previous analysis, that the optical depth for a spherical (E0) halo andfor an oblate (E6) halo are roughly the same, essentially because two competingeffects cancel approximately. However the optical depth for an E6 prolate halois reduced by ~35%. This means that an all-Macho prolate halo with reasonableparameters for the Galaxy is consistent with the published microlensing eventrate.Comment: 7 pages (24K), LaTeX; 2 Postscript figure