Possible γ-ray line from the Crab Nebula

A BALLOON-BORNE γ-ray telescope was flown on 10–11 May 1976 from Alamogordo, New Mexico to search for celestial γ-ray lines in the energy range 100 keV–5.0 MeV. We report here evidence for a line feature from the Crab Nebula at 400±1 keV. Although the counting statistics do not allow us to make an incontestible claim, the fact that it occurs at a reasonable energy to be gravitationally redshifted positron annihilation radiation occurring at the surface of the Crab Pulsar, and in all respects seems to be a real astronomical line, has encouraged us to bring this to the attention of the astronomical community.