Observation of the Suppression of the Flux of Cosmic Rays above<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>4</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>19</mml:mn></mml:msup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>eV</mml:mi></mml:math>

Abstract

The energy spectrum of cosmic rays above $2.5\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\mathrm{eV}$, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index $\ensuremath{\gamma}$ of the particle flux, $J\ensuremath{\propto}{E}^{\ensuremath{-}\ensuremath{\gamma}}$, at energies between $4\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\mathrm{eV}$ and $4\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\mathrm{eV}$ is $2.69\ifmmode\pm\else\textpm\fi{}0.02(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.06(\mathrm{syst})$, steepening to $4.2\ifmmode\pm\else\textpm\fi{}0.4(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.06(\mathrm{syst})$ 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.