Violent black holes linked to high-energy cosmic rays

A cosmic ray event viewed by all four of the Auger Observatory's fluorescence detectors. Each detector records the growth and decay of the extensive cosmic ray air shower, comprised of billions of secondary particles.
Source: http://www.auger.org/
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A surface detector station - part of the Auger Observatory - with the Andes in the background.
Source: http://www.auger.org/
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Friday, 9 November 2007

University of Adelaide scientists are among a leading international research group that has made an important discovery about the highest-energy cosmic rays that hit the Earth - and the discovery leads back to supermassive black holes.

The scientists, in the University's School of Chemistry & Physics, are among researchers from 17 countries participating in the Pierre Auger Collaboration, using the largest cosmic ray observatory in the world, the Pierre Auger Observatory in Argentina.

The team's most recent results will appear in the latest issue (November 9) of the international journal Science.

In Science, the Pierre Auger Collaboration announces that Active Galactic Nuclei - thought to be powered by supermassive black holes that devour large amounts of matter - are the most likely candidate for the source of the highest-energy cosmic rays that hit Earth.

Using the Pierre Auger Observatory, the team of scientists found that the sources of the highest-energy particles are not distributed uniformly across the sky. Instead, the Auger results link the origins of these mysterious particles to the locations of nearby galaxies that have active nuclei in their centres.

Active Galactic Nuclei (AGN) have long been considered sites where high-energy particle production might take place. They swallow gas, dust and other matter from their host galaxies and spew out particles and energy.

While most galaxies have black holes at their centre, only a fraction of all galaxies have an AGN. The exact mechanism of how AGNs can accelerate particles to energies 100 million times higher than the most powerful particle accelerator on Earth is still a mystery.

"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 French physicist Pierre Auger in 1938," says Nobel Prize winner James Cronin, of the University of Chicago, who conceived the Pierre Auger Observatory together with Alan Watson of the University of Leeds.

"We find the southern hemisphere sky as observed in ultra-high-energy cosmic rays is non-uniform. This is a fundamental discovery. The age of cosmic-ray astronomy has arrived. In the next few years our data will permit us to identify the exact sources of these cosmic rays and how they accelerate these particles."

Cosmic rays are comprised of protons and atomic nuclei, which travel across the universe at close to the speed of light. When these particles smash into the upper atmosphere of our planet, they create a cascade of secondary particles called an "air shower" that can spread across 40 or more square kilometres as they reach the Earth's surface.

"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," says Associate Professor Bruce Dawson from the University of Adelaide. "The most exciting thing is that the Observatory is only just beginning, so there is huge scope for further discoveries."

The Adelaide research group, led by associate Professor Bruce Dawson and Professor Roger Clay in the University's School of Chemistry & Physics, was a foundation member of the Auger collaboration.

The first design workshop for the Auger Observatory was held at the University of Adelaide in January 1993. Since then, Adelaide scientists have made many key contributions to the experiment during its design and construction stages. With data now being collected, they have assumed leadership roles in data analysis and interpretation for the international group.

The Pierre Auger Observatory records cosmic ray showers through an array of 1600 particle detectors placed 1.5 kilometres apart in a grid spread across 3000 square kilometres. Twenty-four specially designed telescopes record the emission of fluorescence light from the air shower. The combination of particle detectors and fluorescence telescopes provides an exceptionally powerful instrument for this research.

While the observatory has recorded almost a million cosmic-ray showers, only the rare, highest-energy cosmic rays can be linked to their sources, since only these particles can slice through the magnetic fields that fill outer space.

The Auger collaboration discovered that the 27 highest-energy events do not come equally from all directions. Comparing the clustering of these events with the known locations of 381 Active Galactic Nuclei, the collaboration found that most of these events correlated well with the locations of AGNs in some nearby galaxies, such as Centaurus A.

The Pierre Auger Observatory is being built by a team of more than 370 scientists and engineers from 17 countries. The collaboration is a true international partnership in which no country contributed more than 25% of the US$54 million construction cost.

The observatory is named for French scientist Pierre Victor Auger (1899-1993), who in 1938 was the first to observe the extensive air showers generated by the interaction of high-energy cosmic rays with the Earth's atmosphere.

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