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1.5. GZK-end of the cosmic ray spectrum?

A first hint of a puzzle surfaced in the highest energy Fly's Eye event [122] which has no apparent progenitor within the Local Supercluster [123]. Subsequent observations with the AGASA experiment [124] carried strong indication that the cutoff was somehow circumvented in the absence of plausible nearby sources.

The big disappointment of 2002 was the CR-flux reported by the HiRes Collaboration [125, 126], which is in sharp disagreement with AGASA data [127]. The discrepancy between the two estimated fluxes is shown in Fig. 3. One can argue correctly that the statistical significance of the discrepancy is small, although such an assesment requires a conspiracy between the two groups to bend their maximal systematic errors in opposite directions. Moreover, an analysis [130] of the combined data reported by the HiRes, the Fly's Eye, and the Yakutsk collaborations is supportive of the existence of the GZK cutoff at the > 5sigma (> 3.7sigma) level. The deviation from GZK depends on the set of data used as a basis for power law extrapolation from lower energies. An additional input for this analysis was the recent claim [131] that there may be technical problems with the Yakutsk data collection. More recently, fingerprints of super-GZK CRs have been found [36] by reanalyzing the SUGAR data [13]. However, as one can see in Fig. 3, the number of events is not enough to weight in on one side or the other with respect to the GZK question.

Figure 3

Figure 3. Upper end of the cosmic ray energy spectrum as observed by AGASA [127], Fly's Eye [128], Haverah Park [129], HiRes [126], and SUGAR [36].

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