Measurement of the UHECR energy spectrum using the surface detector of the Pierre Auger Observatory

We report a measurement of the cosmic ray energy spectrum based on the high statistics collected by the surface detector of the Pierre Auger Observatory. We described the methods developed to determine the spectrum from reconstructed observables. The methods exploit the combination of fluorescence detector (FD) and surface detector (SD) and do not rely on detailed numerical simulation or any assumption about the chemical composition. The energy calibration of the observables, which exploits the correlation of surface detector data with fluorescence measurements in hybrid events, is presented in detail. Besides presenting statistical uncertainties, we address the impact of systematic uncertainties. We also summarize the combined energy spectrum obtained using the showers detected with zenith angles between 60 ∘ and 80 ∘ , and the hybrid data which extends the spectrum towards lowers energies.