Physics – Nuclear Physics
Scientific paper
Mar 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nuphs..75..109h&link_type=abstract
Nuclear Physics B Proceedings Supplements, Volume 75, Issue 1-2, p. 109-118.
Physics
Nuclear Physics
4
Scientific paper
There is progress, in the sense that although the energies assigned to cosmic rays by air shower arrays may need reducing by about 20%, energy measurements are consistent at about this level, and several experiments now accord with a spectrum which astonishingly shows no GZK cut-off near 1020eV, greatly limiting the possible source regions. The simplest interpretation is that few cosmic rays above 1019eV come from hundreds of Mpc, contrary to expectation on geometrical grounds. Most of the arrival directions of showers above 4x1019eV show little correlation with the supergalaxy or with matter concentrations within 200Mpc. The implications of the spectrum and the arrival directions are discussed. The most likely explanations of their unexpected features are either (a) that these cosmic rays come mainly from the decay of cosmological relic particles clustered in a large galactic halo (though not if photons really do form a large part of the decay spectrum), or (b) that local intergalactic magnetic fields are unexpectedly strong, and disguise the position of and enhance one extraordinary source within a few Mpc, or (c) most particles above 1019eV are very different from protons after all, and do not have a threshold for serious energy loss as low as 1020eV, or (d) that there is a conspiracy of matching fluxes, so that most cosmic rays above 1019eV are from widespread extragalactic sources, but a superimposed hard spectrum from decaying halo relic particles neatly reduces the magnitude of the GZK fall-off above 1020eV. Several close pairs of arrival directions may yet turn out to be accidental, but if not, explanations (a) and (b) fail. There are several diagnostic tests to be made by the well-placed first (Southern) Auger Project detector and the High-Resolution Fly's Eye detector.
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