  We study the impact of bound state formation on dark matter annihilation
rates in models where dark matter interacts via a light mediator, the dark
photon. We derive the general cross section for radiative capture into all
possible bound states, and point out its non-trivial dependence on the dark
matter velocity and the dark photon mass. For indirect detection, our result
shows that dark matter annihilation inside bound states can play an important
role in enhancing signal rates over the rate for direct dark matter
annihilation with Sommerfeld enhancement. The effects are strongest for large
dark gauge coupling and when the dark photon mass is smaller than the typical
momentum of dark matter in the galaxy. As an example, we show that for thermal
dark matter the Fermi gamma ray constraint is substantially increased once
bound state effects are taken into account. We also find that bound state
effects are not important for dark matter annihilation during the freeze out
and recombination epochs.

High Energy Physics - Phenomenology;High Energy Astrophysical Phenomena