X-rays from Inelastic Dark Matter Freeze-in

We study inelastic dark matter produced via freeze-in through a light mediator with a mass splitting below the electron-positron threshold. In this regime, the heavier dark matter state is naturally long-lived compared to the age of the Universe and decays to the lighter state in association with photons. Given a light mediator, the dark matter abundance is directly related to the decay rate of the heavier dark matter. We show that observations of photons from the galactic center can effectively probe inelastic dark matter freeze-in with mediators at the $100\mathrm{M}\mathrm{e}\mathrm{V}$ scale and dark matter at the $\mathrm{G}\mathrm{e}\mathrm{V}$ scale.