Accurate Inverse-Compton Models Strongly Enhance Leptophilic Dark Matter Signals

The annihilation of TeV-scale leptophilic dark matter into electron-positron pairs (hereafter $e^{+}{e}^{-}$) will produce a sharp cutoff in the local cosmic-ray $e^{+}{e}^{-}$ spectrum at an energy matching the dark matter mass. At these high energies, $e^{+}{e}^{-}$ cool quickly due to synchrotron interactions with magnetic fields and inverse-Compton scattering with the interstellar radiation field. These energy losses are typically modelled as a continuous process. However, inverse-Compton scattering is a stochastic energy-loss process where interactions are rare but catastrophic. We show that when inverse-Compton scattering is modelled as a stochastic process, the expected $e^{+}{e}^{-}$ flux from dark matter annihilation is about a factor of $\sim$2 larger near the dark matter mass than in the continuous model. This greatly enhances the detectability of heavy dark matter annihilating to $e^{+}{e}^{-}$ final states.