Faint Dark Matter Annihilation Signals and the Milky Way's Supermassive Black Hole

A wide range of mechanisms predict present-day s-wave dark matter (DM) annihilation cross sections that are orders of magnitude below current experimental sensitivity. We explore the capability of DM density spikes around the Milky Way's supermassive black hole to probe such faint signals of DM annihilations, considering a range of possible spike and halo distributions. As an exemplar of a theory with a suppressed s-wave annihilation cross section, we consider a hidden sector axion portal model of DM. In this model, the leading contribution to the annihilation cross section in the early Universe is p-wave, while s-wave annihilations occur at higher order in the coupling constant. We provide a unified treatment of DM freeze-out in this model including both s- and p-wave annihilations and analytically determine the photon spectrum for the dominant DM annihilation process in the Universe today. We find that Fenni and H.E.S.S. observations of the Galactic center offer excellent sensitivity to this model over a wide range of parameter space, with prospects depending sensitively on the properties of the DM spike as well as the central halo.