November 13, 2007
November 8, 2007
From C — Cosmic rays, enigmatic particles that flit through the universe, are intriguing for many reasons. The fastest ones are much more energetic than anything that an earthbound accelerator, or indeed any astronomical process that we can convincingly model, can produce. They also have to come from nearby, in intergalactic terms, due to space becoming opaque to them above a certain energy (the GZK cutoff). So what’s making them?
One way to try and find out is to observe a number of them and point them back to their origins, checking to see if they match up with any conceivable source objects. The Pierre Auger Cosmic Ray Observatory has done just that, and has a new result out on the distribution of the highest energy cosmic ray events it has detected (above 5.7 × 1019 eV), showing an anisotropic distribution of the directions of the particles:
(In the plot, the blue is the part of the sky that Auger can see, the black ovals are individual cosmic ray events, and the red stars are the locations of active galactic nuclei (AGN) — galaxies with active supermassive black holes in their centers — within 75 megaparsecs.) In particular, when cross-correlated with a database of AGN, they see many more events within three degrees of known objects than they would expect from a flat distribution. The choice of object to correlate with was made on a pilot sample and their final statistical significance comes from a second, independent dataset, making this sort of a grey box analysis.
Now, apparently, the question is how the AGNs can generate the extreme energies involved…