Dark matter annihilation in substructures revised

Upcoming gamma-ray satellites will search for Dark Matter (DM) annihilations in Milky Way substructures (or 'clumps'). The prospects for detecting these objects strongly depend on the assumptions made on the distribution of DM in substructures, and on the distribution of substructures in the Milky Way halo. By adopting simplified, yet rather extreme, prescriptions for these quantities, we compute the number of sources that can be detected with upcoming experiments such as GLAST, and show that, for the most optimistic particle physics setup (m(chi) = 40 GeV and annihilation cross-section sigma v = 3 x 10(-26) cm(3) s(-1)), the result ranges from zero to approximately 100 sources, all with mass above 10(5) M-circle dot. However, for a fiducial DM candidate with mass m(chi) = 100 GeV and sigma v = 10(-26) cm(3) s(-1), at most a handful of large mass substructures can be detected at 5 sigma, with a one-year exposure time, by a GLAST-like experiment. Scenarios where micro-clumps (i.e. clumps with mass as small as 10(-6) M-circle dot) can be detected are severely constrained by the diffuse gamma-ray background detected by EGRET.