Observation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above $8 \times 10^{18}$ eV

TL;DR

This study reports a significant large-scale anisotropy in the arrival directions of ultra-high energy cosmic rays, suggesting an extragalactic origin based on data from the Pierre Auger Observatory.

Contribution

First detection of a large-scale anisotropy in cosmic ray arrival directions above 8×10^{18} eV using extensive observational data.

Findings

Anisotropy detected at >5.2σ significance

Dipole amplitude of approximately 6.5%

Direction points to extragalactic sources

Abstract

Cosmic rays are atomic nuclei arriving from outer space that reach the highest energies observed in nature. Clues to their origin come from studying the distribution of their arrival directions. Using $3 \times 10^4$ cosmic rays above $8 \times 10^{18}$ electron volts, recorded with the Pierre Auger Observatory from a total exposure of 76,800 square kilometers steradian year, we report an anisotropy in the arrival directions. The anisotropy, detected at more than the 5.2$\sigma$ level of significance, can be described by a dipole with an amplitude of $6.5_{-0.9}^{+1.3}$% towards right ascension $\alpha_{d} = 100 \pm 10$ degrees and declination $\delta_{d} = -24_{-13}^{+12}$ degrees. That direction indicates an extragalactic origin for these ultra-high energy particles.