Research Papers in Physics and Astronomy


Date of this Version

August 2008


Published in PRL 101, 061101 (2008). Copyright 2008. Used by permission.


The energy spectrum of cosmic rays above 2.5 x 1018 eV, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index γ of the particle flux, J/ E- γ , at energies between 4 x 1018 eV and 4 x 1019 eV is 2.69 ± 0.02(stat) ± 0.06(syst), steepening to 4.2 ± 0.4(stat) ± 0.06(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.

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