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

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 x 10⁴ cosmic rays with energies above 8 x 10¹⁸ electron volts, recorded with the Pierre Auger Observatory from a total exposure of 76,800 km² sr year, we determined the existence of anisotropy in arrival directions. The anisotropy, detected at more than a 5.2 level of significance, can be described by a dipole with an amplitude of 6.5–0.9+1.3 percent toward right ascension α_d = 100 ± 10 degrees and declination _d = –24–13+12 degrees. That direction indicates an extragalactic origin for these ultrahigh-energy particles.