The Microlensing and Halo Models of the Galaxy

We investigated the dependence of the optical depth $\tau$ of the microlensing events on model parameters of the Galactic halo. We only consider Galactic mass models in which the rotation curve inside the Sun is compatible with the observation and LMC is bound to the Galaxy. It is found that $\tau$ varies up to a factor 2.5 from the standard spherical and flat rotation halo model. This implies that only the most centrally concentrated halo model can be consistent with the observation if the halo consists of only MACHOs. We also calculate the power $x$ of IMF of MACHO consistent with Tyson's CCD survey as well as Bahcall \etal's observation by {\em HST}. It is found that $x$ is greater than 5.