Measuring the spectrum of UHECR with the Pierre Auger Observatory

The Pierre Auger Observatory is an experiment aimed at the measurement of Extensive Air Showers produced by the Ultra-High Energy Cosmic Rays. It consists of a surface array, which measures the shower particles at ground level, and a fluorescence detector, which observes the development of showers above the array. The measurement of the spectrum of Ultra-High Energy Cosmic Rays using fluorescence observations in coincidence with at least one water Cherenkov detector of the array is reported. In particular, the derivation of the corresponding exposure, which is the main ingredient of the calculation of this spectrum, is discussed. The spectrum based only on the surface array data is also described. Finally, the combination of the two spectra, as well as the systematic uncertainties of the combined spectrum is addressed.