Measurement of the cosmic ray spectrum above 4 × 10¹⁸ eV using inclined events detected with the Pierre Auger Observatory

Abstract

A measurement of the cosmic-ray spectrum for energies exceeding 4×10¹⁸ eV is presented, which is based on the analysis of showers with zenith angles greater than 60° detected with the Pierre Auger Observatory between 1 January 2004 and 31 December 2013. The measured spectrum confirms a flux suppression at the highest energies. Above 5.3×10¹⁸ eV, the "ankle", the flux can be described by a power law E^-γ with index γ=2.70 ± 0.02 (stat) ± 0.1 (sys) followed by a smooth suppression region. For the energy (E<inf>s</inf>) at which the spectral flux has fallen to one-half of its extrapolated value in the absence of suppression, we find E<inf>s</inf>=(5.12 ± 0.25 (stat)^+1.0<inf>-1.2</inf> (sys))×10¹⁹ eV.