Clustering of ultrahigh energy cosmic rays and their sources

The sky distribution of cosmic rays with energies above the “GZK cutoff” holds important clues to their origin. The AGASA data, although consistent with isotropy overall, shows evidence for small-angle clustering, and it has been argued that such clusters are aligned with BL Lacertae objects, implicating these as the sources. It has also been suggested that such clusters can arise if the cosmic rays come from the decays of very massive relic particles in the galactic halo, due to the expected clumping of cold dark matter. We examine these claims and show that both are, in fact, unjustified.