Measurement of the Energy Spectrum of Ultra- High Energy Cosmic Rays using Hybrid Data of

The subject of this thesis is themeasurement of the energy spectrumof ultra-high energy cos-mic rays from simultaneous observation of fluorescence and surface detectors of the Pierre Auger Observatory. These hybrid data allow us to study a wide energy range, which in-cludes the region were the transition between galactic and extragalactic cosmic rays is ex-pected. Dedicated event selection criteria result in an unprecedented energy resolution of less then 10%, which makes a precise determination of spectral features possible. To deter-mine the exposure accumulated during the first years of operation of the observatory, a novel technique has been developed. Based on a very detailed description of the data taking con-ditions and the status of all parts of the Pierre Auger Observatory, a time dependent Monte Carlo simulation has been introduced. Extensive comparisons ensure the exact reproduction of actual data at all levels and time scales. The definition of an energy dependent fiducia