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

Cosmic rays are high-energy particles arriving from space; some have energies far beyond those that human-made particle accelerators can achieve. The sources of higher-energy cosmic rays remain under debate, although we know that lower-energy cosmic rays come from the solar wind. The Pierre Auger Collaboration reports the observation of thousands of cosmic rays with ultrahigh energies of several exa–electron volts (about a Joule per particle), arriving in a slightly dipolar distribution (see the Perspective by Gallagher and Halzen). The direction of the rays indicates that the particles originated in other galaxies and not from nearby sources within our own Milky Way Galaxy.

Science, this issue p. 1266; see also p. 1240

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 with energies above 8 × 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 5.2σ level of significance, can be described by a dipole with an amplitude of ${{\displaystyle 6.5}}_{-0.9}^{+1.3}$ percent toward right ascension α_d = 100 ± 10 degrees and declination δ_d = ${{\displaystyle -24}}_{-13}^{+12}$ degrees. That direction indicates an extragalactic origin for these ultrahigh-energy particles.