Flaring black holes may solve cosmic ray puzzle

WHERE do ultra high-energy cosmic rays come from? These charged particles zoom to Earth from outer space, but why is a mystery. Now a possible source - gamma-ray bursts, which seemed to have been ruled out - have received a new lease of life.

Gamma-ray bursts are usually created by exploding stars, which produce neutrinos. So last April, when the IceCube neutrino detector in Antarctica saw no neutrinos accompanying high-energy cosmic rays, astronomers favoured galaxies with active supermassive black holes at their cores as the source of the rays.

But a more recent study found that only one galaxy was powerful enough to have produced cosmic rays with such high energies. The rest appear to come from galaxies that seem too weak.

This posed a "perplexing problem", says Glennys Farrar of New York University, one of the study authors. Then they found a clue: gamma-ray burst GRB110328A, which happened in March 2011. Its afterglow persisted for over a week, instead of a few hours like normal ones. The culprit was most likely a star falling into a galaxy's central black hole. This would make a weak black hole flare up, producing a burst of gamma rays that in turn spits out cosmic rays, suggests Farrar (arxiv.org/abs/1207.3186v1).

The trouble is testing the hypothesis. Gamma rays travel at the speed of light, so would arrive millennia ahead of any cosmic rays. Farrar hopes to strengthen the idea by matching more cosmic ray emissions with weak active galaxies.

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