Constraining Dark Matter Microphysics with the Annihilation Signal from Subhalos

TL;DR

This paper demonstrates how gamma-ray photon count distributions can reveal the velocity dependence of dark matter annihilation cross sections in subhalos, offering a new way to constrain dark matter microphysics with current and future data.

Contribution

It introduces a method to use gamma-ray PDF analysis to distinguish dark matter annihilation models with different velocity dependencies, including s-wave and Sommerfeld enhancement.

Findings

Current Fermi data can potentially differentiate dark matter models based on velocity dependence.

Gamma-ray PDFs contain information about the microphysics of dark matter annihilation.

Degeneracies exist between velocity dependence, subhalo mass, and signal normalization, but can be broken with priors.

Abstract

In the cold dark matter scenario, galactic dark matter halos are populated with a large number of smaller subhalos. Previous work has shown that dark matter annihilations in subhalos can generate a distinctive, non-Poisson signal in the gamma-ray photon counts probability distribution function (PDF). Here we show that the gamma-ray PDF also carries information about the velocity dependence of the dark matter annihilation cross section. After calculating the PDF assuming $s$-wave and Sommerfeld-enhanced annihilation, we perform a mock data analysis to illustrate how current and future observations can constrain the microphysics of the dark matter annihilation. We find that, with current Fermi data, and assuming a dark matter annihilation cross section roughly at the limit of current bounds from annihilation in dwarf spheroidal galaxies, one can potentially distinguish the non-Poissonian fluctuations expected from dark matter annihilation in subhalos from Poisson sources, as well as from dark matter models with an incorrect velocity-dependence. We explore how robust these results are to assumptions about the modeling of astrophysical backgrounds. We also point out a four-parameter degeneracy between the velocity dependence of the dark matter annihilation, the minimum subhalo mass, the power law index of the subhalo mass function, and the normalization of the dark matter signal. This degeneracy can be broken with priors from N-body simulations or from observational constraints on the subhalo mass function.