Effects of Velocity-Dependent Dark Matter Annihilation on the Energy Spectrum of the Extragalactic Gamma-ray Background

Understand

We calculate the effects of velocity-dependent dark matter annihilation cross sections on the intensity of the extragalactic gamma-ray background.

• Our formalism does not assume a locally thermal distribution of dark matter particles in phase space, and is valid for arbitrary velocity-dependent annihilation.
• As concrete examples, we calculate the effects of p-wave annihilation (with the $v$-weighted cross section of $\sigma v=a+bv^2$) on the mean intensity of extragalactic gamma rays produced in cosmological dark matter halos.
• This velocity variation makes the shape of the energy spectrum harder, but this change in the shape is too small to see unless $b/a\agt 10^6$.