Cosmic-ray composition

It has generally been assumed that the most energetic extragalactic cosmic rays (CRs)—with energies ranging from about 10¹⁸ to 10²¹ eV—are mostly protons accelerated in distant active galaxies. But a new paper from the Pierre Auger Observatory challenges that assumption and raises intriguing issues. The 3000-km² observatory in the high plains of western Argentina is studded with particle detectors and fluorescence telescopes that record the showers of secondary particles engendered by ultraenergetic CR primaries hitting the atmosphere. A clue to a high-energy primary’s identity is how far a shower penetrates into the atmosphere before attaining its maximum development. A proton-induced shower penetrates deeper before reaching that maximum than does a shower induced by a heavier nucleus of the same energy. From the shower-maximum depths of several thousand well-measured ultrahigh-energy events, the Auger collaboration finds that iron nuclei appear to become increasingly dominant over protons in the cosmic-ray flux above 10¹⁹ eV. Alternatively, the highest-energy protons might unexpectedly behave more and more like heavy nuclei at collision energies far above anything yet measured in the laboratory. Such behavior would signal new physics beyond particle theory’s standard model. But if the highest-energy cosmic rays really are Fe²⁶⁺ ions from distant galaxies, their arrival directions should be severely scrambled by intervening magnetic fields. That’s hard to reconcile with the apparent correlations between the anisotropic distribution of matter within a few hundred million light-years and the arrival directions of the most energetic CRs reported earlier by the Auger collaboration. Perhaps the iron sources are just a few active galaxies very close by. Stay tuned! (J. Abraham et al. [Auger collaboration], Phys. Rev. Lett. 104, 091101, 2010 .)—Bertram Schwarzschild

PACS: 96.50.sd, 13.85.Tp, 96.50.sb, 98.70.Sa