The presence of this cutoff had been observed by the HiRes cosmic ray array (Observation of the GZK Cutoff by the HiRes Experiment, arXiv:astro-ph/0703099), but had been disputed by the results from the Japanese detector AGASA (Akeno Giant Air Shower Array) which caused excitement when it failed to see the cut-off in data obtained up to 2004. A third experiment, the Pierre Auger Observatory on the plains of the Pampa Amarilla in western Argentina, which started taking data last year, now settled the question:
"If the AGASA had been correct, then we should have seen 30 events [at or above 1020 eV], and we see two," says Alan Watson, a physicist from the University of Leeds, U.K., and spokesperson for the Auger collaboration [source]. According to Watson, the data also suggests that these highest energy rays comprise protons and heavier nuclei, the latter of which don't feel the GZK drag.
The results were announced on the 30th International Cosmic Ray Conference in Merida, Yucatan, Mexico, and had a brief mentioning in Nature. The Nature article also points out that there is prospect of identifying the regions of the sources of the highest energetic particles, but these data are preliminary. "Unless I talk in my sleep, even my wife doesn't know what these regions are", as Watson was quoted in Nature.
And of course, now that there is new data, somebody is around to claim one needs an even larger experiment to understand it: "Now we understand that above the GZK cutoff there are ten times less cosmic rays than we thought 10 years ago, so we may need a detector ten times as big as Auger," says Masahiro Teshima of the Max Planck Institute for Physics in Munich, Germany, who worked on AGASA and is working on the Telescope Array [source].
The recent paper by the Pierre Auger collaboration with more details was on the arxiv last week:
- The UHECR spectrum measured at the Pierre Auger Observatory and its astrophysical implications
T.Yamamoto, for the Pierre Auger Collaboration, arXiv:0707.2638
Abstract: The Southern part of the Pierre Auger Observatory is nearing completion, and has been in stable operation since January 2004 while it has grown in size. The large sample of data collected so far has led to a significant improvement in the measurement of the energy spectrum of UHE cosmic rays over that previously reported by the Pierre Auger Observatory, both in statistics and in systematic uncertainties. We summarize two measurements of the energy spectrum, one based on the high-statistics surface detize. The large sample of data collected so far has led to a significant improvement in the measurement of the energy spectrum of UHE cosmic rays over that previously reported by the Pierre Auger Observatory, both in statistics and in systematic uncertainties. We summarize two measurements of the energy spectrum, one based on the high-statistics surface detector data, and the other of the hybrid data, where the precision of the fluorescence measurements is enhanced by additional information from the surface array. The complementarity of the two approaches is emphasized and results are compared. Possible astrophysical implications of our measurements, and in particular the presence of spectral features, are discussed.
The upper end of the cosmic ray energy spectrum as measured by the Pierre Auger Observatory: The black dots represent data points, the blue and red curves are expectations derived from different models for the composition and energy distribution of the cosmic ray particles, all based on well-established physics including the GZK cutoff mechanism. Two events cannot be understood as stemming from protons, but may well be explained by heavier nuclei. (Figure from T. Yamamoto, The UHECR spectrum measured at the Pierre Auger Observatory and its astrophysical implications, ICRC'07; Credits: Auger Collaboration, technical information)
More plots and data can be found on the websites of the Pierre Auger Observatory.
TAGS: PHYSICS, COSMIC RAYS