The Pierre Auger Observatory has reported its first major result. Designed to study ultrahigh-energy cosmic rays, the 3000-km2 facility studs the high plains abutting the Andes in western Argentina with 1600 water-Cherenkov detectors watched over by four fluorescence telescopes (see figures 1 and 2).
Construction began in 1999 under the leadership of James Cronin (University of Chicago) and Alan Watson (University of Leeds, UK), and the Auger collaboration has been accumulating data since 2004. Now, with 1400 of the 1600 ground-array detectors in full operation, the collaboration has published evidence that the highest-energy cosmic rays appear to be coming from relatively nearby active galactic nuclei (AGNs). 1 Cronin calls it “the beginning of cosmic-ray astronomy.”
Cosmic rays with energies above 1019 eV are very attractive to astronomers. Like photons and neutrinos, they point back to their sources. Unlike photons and neutrinos, cosmic rays are charged particles—either protons...
