Search for Anisotropy of Ultra-High Energy Cosmic Rays with the Telescope Array Experiment

TL;DR

This study analyzes the distribution of ultra-high energy cosmic rays detected by the Telescope Array, finding no significant anisotropy, clustering, or correlations with nearby galaxies, but suggests magnetic deflections could reconcile observations with large-scale structure models.

Contribution

It provides the first comprehensive analysis of UHECR anisotropy and source correlations using the Telescope Array data, including the impact of Galactic magnetic fields.

Findings

Event distributions are consistent with isotropy across all energy thresholds.

No significant clustering or correlations with active galactic nuclei are observed.

Higher-energy events are compatible with both isotropy and large-scale structure models when magnetic deflections are considered.

Abstract

We study the anisotropy of Ultra-High Energy Cosmic Ray (UHECR) events collected by the Telescope Array (TA) detector in the first 40 months of operation. Following earlier studies, we examine event sets with energy thresholds of 10 EeV, 40 EeV, and 57 EeV. We find that the distributions of the events in right ascension and declination are compatible with an isotropic distribution in all three sets. We then compare with previously reported clustering of the UHECR events at small angular scales. No significant clustering is found in the TA data. We then check the events with E>57 EeV for correlations with nearby active galactic nuclei. No significant correlation is found. Finally, we examine all three sets for correlations with the large-scale structure of the Universe. We find that the two higher-energy sets are compatible with both an isotropic distribution and the hypothesis that UHECR sources follow the matter distribution of the Universe (the LSS hypothesis), while the event set with E>10 EeV is compatible with isotropy and is not compatible with the LSS hypothesis at 95% CL unless large deflection angles are also assumed. We show that accounting for UHECR deflections in a realistic model of the Galactic magnetic field can make this set compatible with the LSS hypothesis.