Bremsstrahlung in dark matter annihilation

We show that the energy spectra from dark matter (DM) annihilation into f+f−γ (where f is a light fermion) via chirality preserving interactions are identical in 3 scenarios: (a) DM is a Majorana fermion and the particle exchanged is a scalar, (b) DM is a Majorana fermion and the particle exchanged is a vector and (c) DM is a scalar and the particle exchanged is a fermion. For cases (a) and (c), we also calculate the differential cross section to ℓ+f−V, where V=W,Z,γ, and ℓ is a light fermion that may or not be the same as f. The form of the cross section depends on whether the DM is a Majorana fermion or a scalar but in both cases its form is independent of V.