Evidence for correlated changes in the spectrum and composition of cosmic rays at extremely high energies

Abstract

The Utah Fly's Eye detector has revealed a change in the cosmic ray composition which is correlated with structure in the all-particle energy spectrum. The data can be fitted by a simple model of a steep power law spectrum of heavy nuclei which is overtaken at high energies by a flatter spectrum of protons. The transition occurs near ${10}^{18.5}$ eV. Anisotropy is not detected, so the high-rigidity particles above the transition energy do not originate in the disk of the Galaxy. An outstanding event of 3\ifmmode\times\else\texttimes\fi{}${10}^{20}$ eV implies that the highest energy particles originate in the contemporary era of the Universe.