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>

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...