Preprint number: CP3-Origins-2016-17 DNRF90 and DIAS-2016-17
We introduce a new type of gamma-ray spectral feature, which we denominate gamma-ray triangle. This spectral feature arises in scenarios where the dark matter self-annihilates into two Dirac fermions of the same chirality, which subsequently decay in flight into another fermion and a photon. The resulting photon spectrum resembles a sharp triangle and can be readily searched for in the gamma-ray sky. Using data from the Fermi-LAT and H.E.S.S. instruments, we find no evidence for such spectral feature and therefore set strong upper bounds on the corresponding annihilation cross section. A concrete realisation of a scenario yielding gamma-ray triangles consists of an asymmetric dark matter model where the dark matter particle carries lepton number. We show explicitly that this class of models can lead to intense gamma-ray spectral features, potentially at the reach of upcoming gamma-ray telescopes, opening a new window to explore asymmetric dark matter through indirect searches.