Sub-GeV Dark Matter Annihilation: Limits from Milky Way observations with INTEGRAL

From 16 years of INTEGRAL/SPI $\gamma$-ray observations, we derive bounds on annihilating light dark matter particles in the halo of the Milky Way up to masses of about 300 MeV. We test four different spatial templates for the dark matter halo, including a Navarro-Frenk-White (NFW), Einasto, Burkert, and isothermal sphere profile, as well as three different models for the underlying diffuse Inverse Compton emission. We find that the bounds on the s-wave velocity-averaged annihilation cross sections for both the electron-positron and the photon-photon final states are the strongest to date from $\gamma$-ray observations alone in the mass range $\lesssim 6$ MeV. We provide fitting formulae for the upper limits and discuss their dependences on the halo profile. The bounds on the two-photon final state are superseding the limits from the Cosmic Microwave Background in the range of 50 keV up to $\sim 3$ MeV, showing the great potential future MeV mission will have in probing light dark matter.