Interesting to see how quickly the story on high-energy galactic cosmic rays has shifted in the past week. Recent work at the Pierre Auger Observatory in Argentina pointed strongly to the centers of active galaxies, where supermassive black holes are found, as the likely source. These Active Galactic Nuclei (AGN) stood out in analysis of the 27 highest energy events recorded at the Auger site because known AGNs seemed to correlate (in terms of direction) with the incoming cosmic rays.
In any case, the idea that these tortured galactic centers could be the source made obvious and intuitive sense. But is the origin of these most powerful of cosmic rays — with energies up to 100 x 1018 electronvolts — now understood, or is it just a statistical correlation that won’t stand up to continued scrutiny? The University of Utah-based High Resolution Fly’s Eye (HiRes) collaboration has been trying to check the correlation based on events in northern hemisphere skies. And here’s the gist, as reported by Katherine Sanderson in an article on nature news:
[HiRes] has tried to check this conclusion against data from ultra-high-energy cosmic-ray events they detected in the northern sky. The researchers used the Auger team’s parameters for factors such as the maximum distance of the active galactic nuclei from Earth and the maximum angle the ray will be bent by interactions with magnetic fields, and looked to see whether the active galactic nuclei could also explain their events. Their results suggest that they don’t.
“They see correlations, we do not see correlations,” says Gordon Thomson, one of the HiRes collaborators. Only 2 of the 13 events looked at by the HiRes team correlated with active galactic nuclei.
Although this first check on the Auger findings hasn’t been published yet, we already seem to be in familiar territory. Cosmic rays have taken us down many a crooked path in the past. Whether HiRes is on to something may depend on whether their dataset and Auger’s are comparable, given that HiRes seems to have few events in the highest-energy range. But we won’t know more about the specifics until the paper appears. Even if Auger is eventually confirmed, the deeper question remains: How does even an AGN produce the accelerations needed to produce these cosmic rays? That question should continue to confound us no matter where in the sky we trace the origin of these mysterious high-energy particles.