Highest-energy cosmic rays

The recently completed Pierre Auger cosmic-ray observatory in Argentina covers 3000 km² with ground detectors and fluorescence telescopes. Its purpose is to determine the distribution in energy, composition, and arrival direction of extragalactic cosmic rays with energies above 10¹⁸ eV. Seeking evidence of what accelerates cosmic rays to ultrahigh energies, two groups have reported searches for correlations between the directions of the 27 highest-energy cosmic-ray events reported by Auger last year and the locations of candidate source galaxies (see PHYSICS TODAY, January 2008, page 16). Matthew George (University of Cambridge) and coworkers have found a strong correlation between the arrival directions and the positions of active galactic nuclei closer than 300 million light-years with particularly intense hard x-ray output. The other correlation study, by Gabriele Ghisellini (Brera Astronomical Observatory, Merate, Italy) and coworkers, found a strong correlation between the arrival directions and the positions of galaxies with intense spectral emission indicative of abundant neutral atomic hydrogen and therefore of spiral galaxies with inactive nuclei. That suggests to Ghisellini and company that the sources of ultrahigh-energy cosmic rays may be newly born magnetars (hypermagnetized neutron stars) in nearby spiral galaxies rather than whole galactic nuclei acting as accelerators. (M. R. George et al., http://arxiv.org/abs/0805.2053; G. Ghisellini et al., http://arxiv.org/abs/0806.2393.) — Bertram M. Schwarzschild