Image credit: NASA/GSFC.
Measurements by the Pierre Auger Observatory may provide evidence of natural nuclear accelerators at work in the local galaxy, the Milky Way. Alexander Kusenko of the University of California, Los Angeles, his student Antoine Calvez, and Shigehiro Nagataki, from Kyoto University, have found that possible sources such as gamma-ray bursts (GRBs) or rare types of supernova explosions could produce the observed energy-dependent composition of ultrahigh-energy cosmic rays.
Earlier this year, the Pierre Auger Collaboration published an analysis of cosmic rays with energies above 1018 eV (1 EeV), which indicated a gradual increase in the average mass of the cosmic rays with energy, up to about 59 EeV (Abraham et al. 2010). In other words, these ultrahigh-energy cosmic rays appear to be heavier nuclei, rather than protons. Previous results, such as the lack of anisotropy in their arrival direction have indicated an extragalactic origin for the highest-energy cosmic rays. However, it seemed surprising that nuclei would travel such long journeys without disintegrating into protons. Moreover, it is unlikely that a cosmic accelerator could accelerate nuclei better than protons at these high energies.
Kusenko and colleagues have now proposed an explanation in which the nuclei originate from sources within the Galaxy (Calvez, Kusenko and Nagataki 2010). Stellar explosions, such as GRBs, can accelerate protons and nuclei but, while the protons leave the Galaxy promptly, the heavier and less mobile nuclei become trapped in the turbulent magnetic field of the source, lingering longer than protons. As a result, the local density of nuclei is increased, so they bombard Earth in greater numbers, as seen by the Pierre Auger Observatory. The nuclei detected will have been trapped by Galactic magnetic fields for millions of years, so their arrival directions have been completely randomized. However, protons escaping from other galaxies should still be seen at the highest energies, and should point back to their sources.
Further reading
J Abraham et al. 2010 Phys. Rev. Letts. 104 091101.
A Calvez, A Kusenko and S Nagataki 2010 Phys. Rev. Letts. 105 091101.
