Probing the Large Magellanic Cloud halo for MACHOs

If galactic halos are composed of massive compact halo objects (MACHOs) in the mass range M <~ 0.01 M_sun_, then ongoing gravitational lensing experiments could be capable of detecting them in the Large Magellanic Cloud (LMC). The signature of LMC (as opposed to Galactic) MACHOs is a variation of the Optical depth to lensing as a function of sky position. Assuming that the LMC halo is spherical, the optical depth along lines of sight toward the far side of the LMC disk should be ~20% higher than those toward the near side because the former pass through more of the halo. If the LMC halo is oblate, the effect is diminished as the square of the halo flatness ratio. The amplitude of the spatial variation in the lensing rate can therefore provide information about the flattening of the LMC halo.