511 keV line constraints on feebly interacting particles from supernovae

Feebly interacting particles with masses $\mathcal{O}(10--100)\text{ }\text{ }\mathrm{MeV}$ can be copiously produced by core-collapse supernovae (SNe). In this paper we consider the case of MeV-ish sterile neutrinos and dark photons mixed ordinary photons, respectively. Furthermore, both may decay into positrons on their route to Earth. Such would annihilate electrons in Galactic medium contribute photon flux 511 keV line. Using spectrometer INTEGRAL observation line improves bounds mixing parameters for these several orders magnitude below what is already excluded SN 1987A energy-loss argument.