Primary γ-rays

Abstract

Within our Galaxy, cosmic rays can reveal their presence in interstellar space and probably in source regions by their interactions with interstellar matter which lead to γ-rays with a very characteristic energy spectrum. From the study of the intensity of the high energy γ radiation as a function of galactic longitude, it is already clear that cosmic rays are almost certainly not uniformly distributed in the Galaxy and are not concentrated in the centre of the Galaxy. The galactic cosmic rays appear to be tied to galactic structural features, presumably by the galactic magnetic fields which are in turn held by the matter in the arm segments and the clouds. On the extra-galactic scale, it is now possible to say that cosmic rays are probably not at the density seen near the Earth. The diffuse celestial γ-ray spectrum that is observed presents the interesting possibility of cosmological studies and possible evidence for a residual universal cosmic ray density, which is much lower than the present galactic cosmic-ray density.