Measurement of the energy spectrum of cosmic rays above 1018 eV with the Pierre Auger Observatory

Full text: The measurement of the flux of cosmic rays of energy above 1018 eV using data obtained in the Pierre Auger Observatory is presented. The spectrum is derived from data measured by the surface detector array and also in the hybrid mode, from fluorescence observations in coincidence with at least one surface detector. The latter condition enables the shower geometry and consequently the energy of the primary particle to be determined with higher accuracy. The hybrid spectrum is determined from data taken between November 2005 and May 2008. Using stringent quality selection criteria, the exposure accumulated during this period is computed and the flux of cosmic rays above 1018eV determined. Finally, the spectrum obtained with the surface detector array only, using data until the end of December 2008, is combined with the hybrid one, addressing the systematic uncertainties and the influence of the energy resolution on the spectral shape. The fluxes obtained with the two techniques are in good agreement in the overlapping energy range. The combined spectrum is derived over a wide energy range with unprecedented precision and the highest statistics available. The spectrum can be described by a broken power law E-γ with an index γ 3.3 below the ankle which is measured at log10(Eankle=eV ) = 18.6. Above the ankle the spectrum is described by a power law with index 2.6 followed by a flux suppression, above about log10(Ebreak=eV ) = 19:5, detected with high statistical significance. (author)