Novel Gamma-ray Spectral Features in the Inert Doublet Model

The inert doublet model contains a neutral stable particle which is an excellent dark matter candidate. We discuss in this paper the indirect signatures of this model in gamma-rays when the dark matter mass is larger than the $W$ boson mass. We show that, in addition to the featureless gamma-ray spectrum produced in the annihilations into two weak gauge bosons, the model generically predicts a distinctive spectral feature from the internal bremsstrahlung process $H^0 H^0 \rightarrow W^+ W^- \gamma$. We discuss under which conditions the spectral feature is generated and we construct a number of benchmark points, compatible with the observed relic density and all other direct and indirect detection experiments, which lead to a sharp gamma-ray feature from internal bremsstrahlung.