Monster black holes power highest-energy cosmic rays

19:00 08 November 2007
NewScientist.com news service
Hazel Muir

Enormous black holes in galaxies millions of light years away are pelting us with energetic particles. The finding, from a telescope array 10 times the size of Paris, solves a long-standing mystery about the origins of the most energetic cosmic rays that strike the Earth's atmosphere.

"Finding an association with something in the sky – it's just fantastic," says Alan Watson from the University of Leeds in the UK, a spokesperson for the team that made the discovery. "The result heralds a new window to the nearby universe and the beginning of cosmic-ray astronomy."

Cosmic rays are charged particles such as protons and atomic nuclei that constantly rain down on Earth's atmosphere. Most come from the Sun and other sources within our galaxy, such as supernova remnants.

But the origins of the highest-energy particles, which travel within a whisker of the speed of light, have been puzzling. A single proton can have as much energy as a tennis ball served at 100 kilometres per hour.

Astronomers found it difficult to explain how particles are accelerated to such enormous speeds.

One possibility was that they are spat out by "active galactic nuclei" (AGNs) – energetic galaxies powered by matter swirling onto a supermassive black hole.

This would be feasible if the AGNs lay up to a few hundred million light years away. Theory predicts that energetic cosmic rays from farther afield would lose energy before they reached Earth by interacting with relic radiation from the big bang called the cosmic microwave background.

Giant cosmic ray 'net'

Alternative explanations include gamma-ray bursts, violent explosions that are sometimes thought to signal the collapse of a massive star into a black hole. More bizarrely, the cosmic rays could signal the decay of heavy particles that have been trapped inside weird knots in space-time since the big bang.

Testing any idea has been difficult, however, because the highest-energy cosmic rays are very rare. To catch enough of them to investigate their origins, scientists had to build the largest cosmic-ray catcher in the world – the Pierre Auger Observatory.

The international observatory is an array of 1600 detectors covering 3000 square kilometres of land in Argentina. It began operating in 2004 (watch a video overview of the array).

Read more...