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In the cold dark matter scenario, galactic dark matter halos are populatedwith a large number of smaller subhalos. Previous work has shown that darkmatter annihilations in subhalos can generate a distinctive, non-Poisson signalin the gamma-ray photon counts probability distribution function (PDF). Here weshow that the gamma-ray PDF also carries information about the velocitydependence of the dark matter annihilation cross section. After calculating thePDF assuming $s$-wave and Sommerfeld-enhanced annihilation, we perform a mockdata analysis to illustrate how current and future observations can constrainthe microphysics of the dark matter annihilation. We find that, with currentFermi data, and assuming a dark matter annihilation cross section roughly atthe limit of current bounds from annihilation in dwarf spheroidal galaxies, onecan potentially distinguish the non-Poissonian fluctuations expected from darkmatter annihilation in subhalos from Poisson sources, as well as from darkmatter models with an incorrect velocity-dependence. We explore how robustthese results are to assumptions about the modeling of astrophysicalbackgrounds. We also point out a four-parameter degeneracy between the velocitydependence of the dark matter annihilation, the minimum subhalo mass, the powerlaw index of the subhalo mass function, and the normalization of the darkmatter signal. This degeneracy can be broken with priors from N-bodysimulations or from observational constraints on the subhalo mass function.

Constraining Dark Matter Microphysics with the Annihilation Signal from Subhalos