A new theory for the diffuse gamma - ray background

Abstract

A study is made of a model for the diffuse gamma-ray background in which cosmic-ray interactions with the microwave radiation at early epochs initiate an electron-photon cascade whose end product is the presently observed gamma-ray spectrum. Cosmic-ray protons ( at least those with energies above 10 (^12) eV) are assumed to be of extragalactic origin, and the shape of the cosmic-ray spectrum is assumed to be determined in part by the energy losses on the microwave background, so that the gamma- ray and cosmic-ray spectra can be related. Strong evolution of the cosmic-ray sources is required at a rate which can be determined from the observed cosmic-ray spectrum. The model parameters required to give the observed spectrum are obtained, and the resulting model used to compute the gamma-ray spectra expected, using both analytical and numerical approaches. It is found that the predicted gamma-ray spectrum in the range 10(^5) – 10 (^8) eV shows remarkably good agreement with the observations. However a detailed assessment of the model indicates a possible discrepancy with observation above 10 (^7) eV, although confirmation of experimental results must be awaited before the theory can be rejected.