9 November 2007

Black holes: source of high-energy cosmic rays

Cosmos Online
Supermassive black holes at the centre of nearby galaxies have been confirmed as the origin of immensely powerful cosmic rays that continually pelt the Earth's atmosphere.
Black holes: source of high-energy cosmic rays

Ray source: A NASA image of the peculiar galaxy Centaurus A (NGC 5128). The centre of the galaxy harbours one of the closest active galactic nuclei (AGN) to Earth, at a distance of 11 million light-years. Research at the Auger Observatory has found the source of two high-energy cosmic rays is somewhere within 3° of this object. Credit: NASA

SYDNEY: Supermassive black holes at the centre of nearby galaxies have been confirmed as the origin of immensely powerful cosmic rays that continually pelt the Earth’s atmosphere.

“We have taken a big step forward in solving the mystery of the nature and origin of the highest-energy cosmic rays, first revealed by the French physicist Pierre Auger in 1938,” said astronomer James Cronin of the University of Chicago, USA.

“We find the southern hemisphere sky as observed in ultra-high-energy cosmic rays is non-uniform. This is a fundamental discovery,” added Cronin, who is co-author of a paper reporting the find today in the U.S. journal Science.

Cosmic violence

Cosmic rays are sub-atomic particles, typically protons, which travel through space at near to the speed of light. The most powerful of these particles contain as much as 100 million times the energy of the speeding particles we can create in our most sophisticated particle accelerators.

The Earth’s atmosphere, however, protects us from their more harmful effects. When they hit the upper atmosphere a cascade of secondary particles known as an ‘air shower’ is created that can spread across 40 square kilometres at the planet’s surface.

The origin of these rays has remained mysterious since they were first discovered by Auger in the 1930s. Astrophysicists had predicted that Active Galactic Nuclei (AGN) – the violent supermassive black holes found at the centre of some galaxies – could be the source, but there was no evidence to prove this.

Now, experts from 17 countries, have used the newly constructed Pierre Auger Observatory in Malargue, Argentina, to pinpoint AGN as the likely source of the highest-energy cosmic rays hitting Earth.

The team of scientists used particle detectors and fluorescence telescopes to reveal that high-energy cosmic rays do not come equally from all directions. Furthermore, they found that the likely origins of the 27 highest energy events correlated well with clustering of the known locations of 381 AGN in nearby galaxies.

“Spectacular objects”

The experts are still unable to explain how AGN are able to accelerate the cosmic particles to such massive energies, however.

“They are really spectacular objects,” said team member Maximo Ave, of the Kavli Institute for Cosmological Physics in Chicago. “They most likely can be produced only in a place where some very extreme physical process is happening.” One such extreme process might be gamma-ray bursts, the possible result of collapsing or colliding stars, he said.

Only the rarest, most energetic cosmic rays can be traced, said the authors, as less powerful rays are unable to break through the magnetic fields that criss-cross space.

“These enormously energetic particles are very rare, but they pack a real punch. This discovery is a major step towards understanding some of the most extreme processes in the Universe,” said Australian co-author Bruce Dawson at the University of Adelaide. “The most exciting thing is that the Observatory is only just beginning, so there is huge scope for further discoveries.”


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