Arrival Directions of Cosmic Rays above 32 EeV from Phase One of the Pierre Auger Observatory

TL;DR

This paper presents the largest dataset of ultra-high energy cosmic ray arrival directions above 32 EeV from the Pierre Auger Observatory, revealing significant anisotropy at energies above 40 EeV, suggesting potential extragalactic sources.

Contribution

It provides the first extensive dataset of cosmic ray arrival directions at these energies and reports evidence of anisotropy, advancing understanding of cosmic ray origins.

Findings

Over 2,600 cosmic rays above 32 EeV analyzed

Significant anisotropy detected at energies above 40 EeV

Anisotropy significance exceeds 4 sigma

Abstract

A promising energy range to look for angular correlation between cosmic rays of extragalactic origin and their sources is at the highest energies, above few tens of EeV ($1\:{\rm EeV}\equiv 10^{18}\:$eV). Despite the flux of these particles being extremely low, the area of ${\sim}\:3{,}000 \: \text{km}^2$ covered at the Pierre Auger Observatory, and the 17-year data-taking period of the Phase 1 of its operations, have enabled us to measure the arrival directions of more than 2,600 ultra-high energy cosmic rays above $32\:\text{EeV}$. We publish this data set, the largest available at such energies from an integrated exposure of $122{,}000 \: \text{km}^2\:\text{sr}\:\text{yr}$, and search it for anisotropies over the $3.4\pi$ steradians covered with the Observatory. Evidence for a deviation in excess of isotropy at intermediate angular scale, with ${\sim}\:15^\circ$ Gaussian spread or ${\sim}\:25^\circ$ top-hat radius, is obtained at the $4\:\sigma$ significance level for cosmic-ray energies above ${\sim}\:40\:\text{EeV}$.