Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8 × 1018 eV

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 3×104 cosmic rays above 8×1018 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 5.2σ level of significance, can be described by a dipole with an amplitude of 6.5+1.3−0.9% towards right ascension αd=100±10 degrees and declination δd=−24+12−13 degrees. That direction indicates an extragalactic origin for these ultra-high energy particles.