A New Limit on the Distances of Nuclei UHECRs Sources

Abstract

Recent evidence from the Pierre Auger Observatory suggests a transition, at 5 EeV-10EeV in the composition of Ultra High Energy Cosmic Rays (UHECRs), from protons to heavier nuclei such as iron. I consider here the implications of the heavier composition on the sources of UHECRs. The smaller magnetic rigidity implies that nuclei UHECRs are: (i) More easily accelerated (ii) Local, as they can diffuse only a few Mpc from their sources before disintegrating (iii) Isotropic, because large deflections in the extra galactic and the galactic magnetic fields erase the directional information. Uncertainties in the strength and structure of the the extra galactic magnetic field (EGMF) makes it difficult to estimate the overall effects. However, with typical reasonable parameters of a few nG and a coherence distance of a Mpc the distance a nuclei UHECR above the GZK energy traverses before photodisintegrating is only a few Mpc. In spite of the significantly weaker limits on the luminosity, Cen A is the only currently active potential source of nuclei UHECRs within this distance. The large deflections erases the directional anisotropy expected from a single source. If indeed the composition of above-GZK-UHECRs is iron and if the EGMF is not too small then Cen A is the dominant source of observed nuclei UHECRs above the GZK limit.