Large-scale Cosmic-Ray Anisotropies above 4 EeV Measured by the Pierre Auger Observatory

We present a detailed study of the large-scale anisotropies of cosmic rays with energies above 4EeV measured using the Pierre Auger Observatory. For the energy bins [4, 8]EeV and E…8 EeV, the most significant signal is a dipolar modulation in R.A. at energies above 8EeV, as previously reported. In this paper we further scrutinize the highest-energy bin by splitting it into three energy ranges. We find that the amplitude of the dipole increases with energy above 4EeV. The growth can be fitted with a power law with index β=0.79±0.19. The directions of the dipoles are consistent with an extragalactic origin of these anisotropies at all the energies considered. Additionally, we have estimated the quadrupolar components of the anisotropy: they are not statistically significant. We discuss the results in the context of the predictions from different models for the distribution of ultrahigh-energy sources and cosmic magnetic fields.