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 > 5 (> 3.7) 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. 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].