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O rbiting W ide-angle L ight-collectors (OWL)
(click on any OWL to reach home)
The cosmic ray observations above 1017 eV reported by the Fly's Eye, Haverah Park, AGASA, and HiRes show a break in the spectrum at ~5 x 1018 eV. The composition changes from being predominantly heavy nuclei below the break to light nuclei above the break. If these particles were produced in the galaxy, they should be anisotropic because their gyroradii in the few microgauss galactic magnetic fields are bigger than galactic scales. Thus the spectral change, composition, and the observed arrival directions are strongly suggestive that the cosmic rays above 1019 eV are predominantly extra-galactic particles.
If the particles observed are initiated by protons of extra-galactic origin, and if their sources are correlated with luminous matter, then the inhomogeneity of the large scale galaxy distribution, on scales < 100 h-1 Mpc, should be imprinted on their arrival directions. The expected anisotropy associated with the large scale structure should be apparent once the number of events detected above 1019 eV is increased by an order of magnitude.
In any case, the arrival direction distribution will be an important clue in determining their origin. OWL will provide a statistically significant study of the arrival directions of particles beyond the GZK cutoff.
Physics objectives of the OWL include measuring the anisotropy, energy spectrum, and composition of the highest energy cosmic rays. Composition analysis should include the identification of gamma rays or neutrinos if they are present.