Predictions of Extragalactic Gamma Ray Fluxes between 1 and 100 MeV

Abstract

RECENT theoretical studies have shown the need for observations of isotropic cosmic gamma radiation in the 1–100 MeV energy region. Such observations will enable us to determine the relative importance of the various processes which may produce the observed isotropic X-radiation below 1 MeV and gamma radiation above 100 MeV. The first process to be examined as a possible explanation for the radiation below 1 MeV was Compton interactions between metagalactic cosmic ray electrons and photons of the universal microwave radiation field1–4. Recently, we discussed additional processes which may be important in producing extragalactic gamma radiation. These processes are cosmic ray electron bremsstrahlung with intergalactic matter5, collisions of cosmic ray nuclei with intergalactic matter which result in the production of pi-mesons6,7, and the mutual annihilation of matter and antimatter on possible boundary regions of baryon inhomo-geneity in the universe8. Our purpose here is to discuss some implications of these studies in distinguishing the gamma ray spectra produced by the various processes and placing upper limits on the metagalactic cosmic ray electron and nuclear fluxes. In particular, the results indicate that observational studies of isotropic gamma radiation between 1 and 100 MeV are critical for determining the dominant processes producing gamma rays in the metagalaxy and their cosmological implications.