Large-scale distribution of arrival directions of cosmic rays detected above 10¹⁸ ev at the Pierre Auger observatory

Abstract

A thorough search for large-scale anisotropies in the distribution of arrival directions of cosmic rays detected above 10¹⁸ eV at the Pierre Auger Observatory is presented. This search is performed as a function of both declination and right ascension in several energy ranges above 10¹⁸ eV, and reported in terms of dipolar and quadrupolar coefficients. Within the systematic uncertainties, no significant deviation from isotropy is revealed. Assuming that any cosmic-ray anisotropy is dominated by dipole and quadrupole moments in this energy range, upper limits on their amplitudes are derived. These upper limits allow us to test the origin of cosmic rays above 10¹⁸ eV from stationary Galactic sources densely distributed in the Galactic disk and predominantly emitting light particles in all directions.