Reviving MeV-GeV Indirect Detection with Inelastic Dark Matter

Thermal relic dark matter below $\sim 10 \ \text{GeV}$ is excluded by cosmic microwave background data if its annihilation to visible particles is unsuppressed near the epoch of recombination. Usual model-building measures to avoid this bound involve kinematically suppressing the annihilation rate in the low-velocity limit, thereby yielding dim prospects for indirect detection signatures at late times. In this work, we investigate a class of cosmologically-viable sub-GeV thermal relics with late-time annihilation rates that are detectable with existing and proposed telescopes across a wide range of parameter space. We study a representative model of inelastic dark matter featuring a stable state $\chi_1$ and a slightly heavier excited state $\chi_2$ whose abundance is thermally depleted before recombination. Since the kinetic energy of dark matter in the Milky Way is much larger than it is during recombination, $\chi_1 \chi_1 \to \chi_2 \chi_2$ upscattering can efficiently regenerate a cosmologically long-lived Galactic population of $\chi_2$, whose subsequent coannihilations with $\chi_1$ give rise to observable gamma-rays in the $\sim 1 \ \text{MeV} - 100 \ \text{MeV}$ energy range. We find that proposed MeV gamma-ray telescopes, such as e-ASTROGAM, AMEGO, and MAST, would be sensitive to much of the thermal relic parameter space in this class of models and thereby enable both discovery and model discrimination in the event of a signal at accelerator or direct detection experiments.