No 'cut off' in the High Energy Cosmic Ray Energy Spectrum

Abstract

It is often claimed that there should be a 'GZK cut-off" in the flux of extragalactic cosmic rays, arising from interactions between the cosmic rays and the cosmic micro-wave background photons (e.g. 1 and 2). Some experiments (3 and 4) show particles of even higher energy than this value and this has led to claims for exotic processes (e.g. 5 and 6). We contend that such claims are unnecessary - there is no predicted cut-off, rather a continuation of the injection spectrum at reduced intensity. We have combined the world's data and shown that the prediction for a rather flat injection spectrum (exponent: 1.9 - 2.2) in the case of universal particle injection provides a reasonable fit to the data. Conventional forms for the particle attenuation in the intergalactic medium (e.g. 7 and 8) have been assumed. Either protons or iron nuclei (or a mixture) will suffice. Attention is drawn to another aspect, too, that of the losses on the infra-red radiation which may be intense near to strong sources and for sources in galaxy clusters. The attendant magnetic fields near the sources leads to significantly long diffusion times through the strong infra-red fields. Two 'case histories' are considered.