The energy spectrum, composition and arrival directions of ultrahigh energy
cosmic rays (UHECRs) with energy above the cosmic ray ankle, measured by the
Pierre Auger Observatory, are inconsistent if their origin is assumed to be
extragalactic. Their observed properties, however, are those expected from
UHECRs accelerated by the highly relativistic jets emitted in Galactic gamma
ray bursts, most of which are beamed away from Earth. If this alternative
interpretation is correct, the observed break in the energy spectrum of UHECRs
around 50 EeV is not the Greisen-Zatsepin-Kuzmin cutoff but the energy
threshold for free escape of ultrahigh energy iron cosmic rays from the Galaxy
and above their respective free-escape threshold-energies, UHECR nuclei should
point back to their Galactic sources or their remnants rather than to active
galactic nuclei (AGN) within the GZK horizon.Comment: Invited talk presentet by A. Dar at the Rencontre de La Thuile, 2011.
To be published in Nuovo Ciment