Polynomial Spectrum of Gamma Ray from Dark Matter

In this study, we present some general features of gamma-ray spectra from dark matter. We find that the spectrum with sharp features could appear in a wide class of dark matter models and mimic the gamma line signals. If all other physical degrees of freedom are heavy or effectively decoupled, the resulting gamma ray from dark matter decay or annihilation would generally have polynomial-type spectra or power-law with positive index. We illustrate our findings in a model-independent framework with generic kinematic analysis. Similar results can also apply for cosmic ray or neutrino cases.