Electron-positron Annihilation Lines and Decaying Sterile Neutrinos

If massive sterile neutrinos exist, their decays into photons and/or electron-positron pairs may give rise to observable consequences. We consider the possibility that MeV sterile neutrino decays lead to the diffuse positron annihilation line in the Milky Way center, and we thus obtain bounds on the sterile neutrino decay rate $\Gamma_e \ge 10^{-28}$ s$^{-1}$ from relevant astrophysical/cosmological data. Also, we expect a soft gamma flux of $1.2 \times 10^{-4}-9.7 \times 10^{-4}$ ph cm$^{-2}$ s$^{-1}$ from the Milky Way center which shows up as a small MeV bump in the background photon spectrum. Furthermore, we estimate the flux of active neutrinos produced by sterile neutrino decays to be $0.02-0.1$ cm$^{-2}$ s$^{-1}$ passing through the earth.