Speaker
Description
The Pierre Auger Observatory measures several characteristics of ultra-high-energy cosmic rays (UHECRs), in particular the energies, the shower maximum depths of the air-shower profiles, and the arrival directions. Using the energy spectrum and the distributions of shower maximum depth 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 on the 4.5 sigma significance level. The energy-dependency of the arrival directions at the highest energies is well described by the modeled contribution from the starburst catalog or by that of the nearby radio galaxy Centaurus A.
