  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.
