Anisotropy Studies with the Pierre Auger Observatory

TL;DR

The Pierre Auger Observatory detected a significant anisotropy in ultra-high energy cosmic rays, correlating their arrival directions with nearby extragalactic sources, supporting the extragalactic origin hypothesis and consistent with the GZK effect.

Contribution

This study provides the first strong anisotropy signal linking UHECRs to nearby extragalactic sources, advancing understanding of cosmic ray origins.

Findings

Anisotropy signal with >99% confidence level

Correlation with nearby active galactic nuclei

Energy-dependent evolution consistent with GZK effect

Abstract

An anisotropy signal for the arrival directions of ultra-high energy cosmic rays (UHECR) of more than 99% confidence level was established using data collected by the Pierre Auger Observatory. Cosmic rays with energy above $\sim 6 \times 10^{19}$ eV show a correlation with the positions of extragalactic nearby active galactic nuclei (AGN), being maximum for sources at less than $\sim$100 Mpc and angular separation of a few degrees. The evolution of the correlation signal with the energy shows that the departure from anisotropy coincides with the flux suppression observed in the spectrum, being therefore consistent with the hypothesis that the correlated events have their origin in extragalactic sources close enough to avoid significant interaction with the cosmic microwave background (the Greisen-Zatsepin-Kuz'min effect). Even though the observed signal cannot unambiguously identify AGNs as the production sites of UHECRs, the potential sources have to be distributed in a similar way. A number of additional statistical tests were performed in order to further understand the nature of the correlation signal.