Extension of the Cosmic-Ray Energy Spectrum beyond the Predicted Greisen-Zatsepin-Kuz'min Cutoff

Abstract

The cosmic-ray energy spectrum above ${10}^{18.5}\mathrm{eV}$ is reported using the updated data set of the Akeno Giant Air Shower Array from February 1990 to October 1997. The energy spectrum extends beyond ${10}^{20}\mathrm{eV}$ and the energy gap between the highest energy event and the others is being filled up with recently observed events. The spectral shape suggests the absence of the 2.7 K cutoff in the energy spectrum or a possible presence of a new component beyond the 2.7 K cutoff.