Extragalactic gamma-ray signal from Dark Matter annihilation: a power
  spectrum based computation
  We revisit the computation of the extragalactic gamma-ray signal from
cosmological dark matter annihilations. The prediction of this signal is
notoriously model dependent, due to different descriptions of the clumpiness of
the dark matter distribution at small scales, responsible for an enhancement
with respect to the smoothly distributed case. We show how a direct computation
of this "flux multiplier" in terms of the nonlinear power spectrum offers a
conceptually simpler approach and may ease some problems, such as the
extrapolation issue. In fact very simple analytical recipes to construct the
power spectrum yield results similar to the popular Halo Model expectations,
with a straightforward alternative estimate of errors. For this specific
application, one also obviates to the need of identifying (often
literature-dependent) concepts entering the Halo Model, to compare different
simulations.
