"The most energetic particles in the universe have regained some of their former mystery. Last year, it seemed that the origin of these particles had finally been tracked down to a set of giant black holes in nearby galaxies, but a new study casts doubt on that conclusion.
Ultra-high-energy cosmic rays, or UHECRs, are individual sub-atomic particles with energies up to about 1020 electron volts, far beyond anything achieved in particle accelerators.
When they hit the Earth's atmosphere, they produce a shower of other particles, and the Pierre Auger Observatory in Argentina has spotted more of these events than any other detector.
In November 2007, an Auger team looked at the arrival directions of the 27 highest-energy cosmic rays, and found that they fit a suggestive pattern. Most came from within 3° of the directions of nearby active galaxies, which hold supermassive black holes at their cores and emit many kinds of radiation. So it seemed that the galaxies were emitting UHECRs too.
Now researchers led by Igor Moskalenko of Stanford University in California, US, have looked more closely at these particular active galaxies, as well as others along the same line of sight. They find that they are an unremarkable bunch. "The sample consists mainly of low-power active galaxies," says Moskalenko."
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