Origin of cosmic gamma rays

Abstract

An attempt is made to distinguish origin locations and dominant production mechanisms of cosmic gamma rays. The study adds to information about the cosmic ray progenitors, primarily electrons and protons of 10(^8) – 10(^10) eV. The disc longitude distribution for energies above 100 MeV is unfolded to give the Galactic gamma ray emissivity for radial symmetry or uniform emission along spiral sections. The correlation is reasonable with models based on the molecular hydrogen distribution. Inconsistency with spiral arm positions is found. The Galactic centre region, where gas density is high, is a probable example of a thick target region for gamma ray-producing cosmic rays. The emissivity under these conditions is calculated. To satisfy the proportion of the observed flux interpreted as coming from the Galactic centre region, a cosmic ray intensity greater than that locally is required. This supports a Galactic origin for the cosmic rays in question. The required injection rate is several hundreds of times the local value, and its relationship to magnetic field and gas density is examined. Contributions to the high latitude flux from the Galaxy and discrete extragalactic sources are calculated. It is shown that a large part may result from inverse Compton scattering of electrons diffusing away from the Galactic disc. The diffusion is modelled on the basis of other astro-physical data. With the Galaxy as a guide, and relating gamma ray emission to other properties, the contributions from external normal and radio galaxies are calculated. The total of all contributions is found to be a significant proportion of the observed flux, casting doubt on such cosmological models as are normalised to fit the entire measured spectrum in magnitude and shape. Radio galaxies may provide the bulk of the 1-10 MeV background if there is proportionality between their gamma ray and radio luminosities.