Large scale distribution of ultra high energy cosmic rays detected at the Pierre Auger Observatory with zenith angles up to 80$^\circ$

TL;DR

This study analyzes the large-scale distribution of ultra-high-energy cosmic rays detected at the Pierre Auger Observatory, including high zenith angle events, revealing anisotropy especially above 8 EeV with statistically significant deviations from isotropy.

Contribution

First analysis of cosmic ray arrival directions including events with zenith angles up to 80°, extending previous isotropy studies and confirming anisotropy signals at high energies.

Findings

Significant anisotropy detected above 8 EeV in right ascension.

First harmonic amplitude in right ascension is (4.4 ± 1.0)×10⁻².

Chance probability of observed anisotropy is 6.4×10⁻⁵.

Abstract

We present the results of an analysis of the large angular scale distribution of the arrival directions of cosmic rays with energy above 4 EeV detected at the Pierre Auger Observatory including for the first time events with zenith angle between $60^\circ$ and $80^\circ$. We perform two Rayleigh analyses, one in the right ascension and one in the azimuth angle distributions, that are sensitive to modulations in right ascension and declination, respectively. The largest departure from isotropy appears in the $E > 8$ EeV energy bin, with an amplitude for the first harmonic in right ascension $r_1^\alpha =(4.4 \pm 1.0){\times}10^{-2}$, that has a chance probability $P(\ge r_1^\alpha)=6.4{\times}10^{-5}$, reinforcing the hint previously reported with vertical events alone.