Observation of the Suppression of the Flux of Cosmic Rays above $4\times {10}^{19}\mathrm{eV}$

The energy spectrum of cosmic rays above $2.5\times {10}^{18}\mathrm{eV}$, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index $\gamma$ of the particle flux, $J\propto E^{-\gamma }$, at energies between $4\times {10}^{18}\mathrm{eV}$ and $4\times {10}^{19}\mathrm{eV}$ is $2.69\pm 0.02(\mathrm{stat})\pm 0.06(\mathrm{syst})$, steepening to $4.2\pm 0.4(\mathrm{stat})\pm 0.06(\mathrm{syst})$ at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz’min.

Figure 1

Correlation between $\lg S_{38°}$ and $\lg E_{\mathrm{FD}}$ for the 661 hybrid events used in the fit. The full line is the best fit to the data. The fractional differences between the two energy estimators are inset.

Figure 2

Upper panel: The differential flux $J$ as a function of energy, with statistical uncertainties. Data are listed at [32]. Lower Panel: The fractional differences between Auger and HiRes I data [3] compared with a spectrum with an index of 2.69.