Large scale distribution of arrival directions of cosmic rays detected above $10^{18}$ eV at the Pierre Auger Observatory

TL;DR

This study searches for large-scale anisotropies in cosmic ray arrival directions above 10^18 eV at the Pierre Auger Observatory, finding no significant deviations from isotropy and setting upper limits on dipole and quadrupole anisotropies.

Contribution

It provides the first comprehensive upper limits on large-scale anisotropies of ultra-high-energy cosmic rays, constraining models of their galactic origin.

Findings

No significant anisotropy detected within systematic uncertainties.

Upper limits challenge models of stationary galactic sources emitting light particles.

Results support isotropic distribution consistent with extragalactic origins.

Abstract

A thorough search for large scale anisotropies in the distribution of arrival directions of cosmic rays detected above $10^{18}$ 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 $10^{18}$ 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 challenge an origin of cosmic rays above $10^{18}$ eV from stationary galactic sources densely distributed in the galactic disk and emitting predominantly light particles in all directions.