Diffusion of the highest energy cosmic rays from Virgo

Abstract

The origin of the predominant particle component of cosmic rays is still undecided although at ‘low’ energies (E∼10⁹–10¹⁰ eV) γ-ray data provide circumstantial evidence in favour of an origin in galactic sources¹. The galactic magnetic field deflects trajectories, at least until about 10¹⁸ eV, and almost completely destroys any relationship between the directions from which cosmic rays arrive and the directions to their sources, making the elucidation of origins a problem. Above 10¹⁸ eV, if the primary cosmic rays are mainly protons, as would be expected and as is claimed³, then the absence of large anisotropies associated with the galactic plane strongly suggests that the particles are from extragalactic sources. If they are extragalactic and their sources are distributed widely in the Universe then, because of interactions between the protons and the photons of the 2.7 K relict radiation, the observed spectrum should steepen considerably above several times 10¹⁹ eV. However, the opposite appears to be happening: the measured spectrum flattens above about 10¹⁹ eV (refs 4–6) (see Fig. 2). Several suggestions have been made to account for this paradox, in each case the idea being to generate in extragalactic space a production spectrum which is so ‘flat’ that even after energy losses are taken into account, the resulting spectrum at the Earth has the characteristic observed flattening (for example ref. 7). Here we put forward an alternative model where the observed spectral shape arises in a rather natural way, being due to propagation characteristics in the local supercluster, the bulk of the very energetic particles having originated in its central region.