Fermi-LAT Sensitivity to Dark Matter Annihilation in Via Lactea II Substructure

We present a study of the ability of the Fermi Gamma-ray Space Telescope to detect dark-matter annihilation signals from the Galactic subhalos predicted by the Via Lactea~II N-body simulation. We implement an improved formalism for estimating the boost factor needed to account for the effect of dark-matter clumping on scales below the resolution of the simulation, and we incorporate a detailed Monte Carlo simulation of the response of the Fermi-LAT telescope, including a simulation of its all-sky observing mode integrated over a ten year mission. The results are less optimistic than previous estimates that did not fully account for the variations of the LAT effective area and point-spread function. Nevertheless, for WIMP masses up to about 150~GeV/$c^2$ in standard supersymmetric models a few subhalos could be detectable with $>5$ standard deviations significance and would likely deviate significantly from the appearance of a point source.