Supernova remnants and the origin of cosmic radiation: evidence from low-energy gamma rays

Abstract

Our model, in which cosmic rays are accelerated by shocks in supernova remnants (SNR) and then propagate by diffusion through the galaxy, has been developed and applied to the case of the low-energy (Eγ ⩽ 10 GeV) diffuse gamma ray background. Attention is given to the well-known ‘GeV-excess’ problem, i.e. the gamma ray spectrum from the inner galaxy is flatter (and in the outer galaxy is correspondingly steeper) in the range 1–10 GeV, than would have been expected on the basis of gamma rays coming from the interactions of the cosmic rays observed locally with gas and radiation fields. Attention is also given to the related problem of the ‘cosmic ray gradient’, i.e. the dependence of the low-energy cosmic ray intensity on galactocentric distance; the gradient being deduced from the distribution of >0.1 GeV gamma rays which is flatter than the known SNR distribution.We argue that both problems can be solved in the framework of the conception that SN explosions are the source of the turbulence in the galactic disk leading to a radial dependence of the form of the turbulence spectrum. We contend that the propagation of cosmic rays through the turbulent interstellar medium (ISM) is governed by the so-called anomalous diffusion. This form of diffusion leads to a flatter energy spectrum of cosmic rays in the inner galaxy and a steeper spectrum in the outer galaxy. Anomalous diffusion reduces also the radial gradient of GeV cosmic rays in the inner galaxy; for the outer galaxy we invoke re-entry of cosmic rays from the halo.In the halo, where there are no SNR, the turbulence is driven by cosmic rays themselves and their energy spectrum has to be as steep as in the outer galaxy.