Astrophysical interpretations of the data measured at the Pierre Auger Observatory

Abstract The Pierre Auger Observatory measures several characteristics of ultra-high-energy cosmic rays (UHECRs), in particular the energies, the depths of shower maximum of the air-shower profiles, and the arrival directions. Using the energy spectrum and the distributions of depths of shower maximum in a combined fit, the parameters of homogeneously distributed UHECR sources can be constrained. We find that the data are well reproduced if two extragalactic populations of sources are considered, one emitting a soft spectrum dominating below the ankle, and one with a very hard spectrum and mixed composition dominating at the highest energies. In the case of very strong extragalactic magnetic fields between the closest sources and Earth, the spectral index of the high-energy population can be much softer and even in agreement with the expectations from shock acceleration. When taking into account also the arrival directions, it is revealed that adding a population of nearby starburst galaxies to the homogeneous background leads to an improvement of the model likelihood to the 4 . 5 σ significance level. The energy-dependent arrival directions at the highest energies are well described by flux contributions from the starburst galaxy catalog or the nearby radio galaxy Centaurus A.