Fitting the HiRes Data

We fit the recently released monocular spectra of the High Resolution Fly's Eye (HiRes) experiment with broken power laws to identify features in the ultra-high energy cosmic ray (UHECR) spectrum. These fits find the previously observed feature known as the Ankle at 1018.5 eV, as well as evidence for a suppression at higher energies, above 1019.8 eV. We use the integral spectrum along with the E1/2 test to identify this high energy suppression with the GZK suppression. Finally, we use a model of uniformly distributed extragalactic proton sources together with a phenomenological model of the galactic cosmic ray spectrum to compare the HiRes spectra to what should be expected from the GZK suppression, and to measure how the extragalactic sources must evolve and what the input spectral slope must be to fit the HiRes data