The AUGER collaboration has released a new analysis of their data of the ultra high energetic cosmic rays (UHECR). As reported in Science (Science 9 November 2007: 896-897) They find correlations between the events of highest energies and active galactic nuclei (AGN), and are able to reject the hypothesis of an isotropic distribution of these cosmic rays at a confidence level of 99%. This reliably rules out speculations about the origin of these UHECRs in local, galactic sources. Though it has been expected, until now there was no experimental confirmation that they originate outside our galaxy.
What is really cute about their analysis is that this correlation with AGNs up to a distance of ~100 Mpc is present for UHECR with energies higher than a certain threshold, but not for those with lower energies: the correlation increases abruptly at the energy of about 5.7 x 1019 eV, which coincides with the point on the energy spectrum recently reported from the observatory at which the flux is reduced by ~50%. To understand this feature, recall that the GZK-cutoff (for an introduction see here) predicts that the mean free path of protons drops dramatically with increasing energy after a threshold is crossed, and the proton's energy is high enough to scatter at CMB photons to produce pions. In the energy range above ~8 x 1019 eV, protons that come from farther away than ~90 Mpc should be scattered, loose energy, and could never reach the earth. The vanishing of the correlation with AGNs around this threshold is thus is an independent confirmation of the GZK-cutoff, on which we also reported in July.
Aaron Chou from the AUGER collaboration gave a talk on the recent results today at our previously mentioned workshop. The recording and the slides should be available at the PI website soon.
More Info at the AUGER websites.
Update Nov 9th: The talk is now online, see PIRSA 07110054.