Published online 9 November 2007 | Nature | doi:10.1038/news.2007.233

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High-energy cosmic rays traced to source

Supermassive black-hole galaxies spin out super-energetic particles.

Galaxies with super-massive black holes may create super-energetic particles.NASA/ Aurore Simonnet, Sonoma State University

The highest-energy cosmic rays that bombard Earth have been traced to their source — rare galaxies with supermassive black holes at their centre.

A collaboration of more than 370 scientists working with the Pierre Auger Observatory in Argentina tracked the rays by pointing particle detectors skywards and tracing high-energy hits back to the objects that were most likely to have produced them. These high-energy particles hit Earth's atmosphere with an energy that is 100 million times higher than anything produced by man-made particle accelerators. Unlike lower-energy cosmic rays, which are bent and deflected by magnetic fields in the Universe, high-energy rays whizz through space in a nearly straight line, making it possible to trace them back to their source.

Ultra-high-energy cosmic rays were first detected in 1962. But whatever made these particles was so extreme that it didn't fall within any physics known at that time. Since then, scientists have been determined to solve the mystery of where these super-energetic particles come from.

High-energy cosmic rays are extremely rare, with less than one particle hitting a square kilometre of Earth every hundred years. That has made them hard to study. And although they pass in a nearly straight line through space, it has not been known exactly how much they are deflected by galactic magnetic fields.

Source revealed

The vast Pierre Auger Observatory has 1,600 ground-based particle detectors over an area of 3,000 square kilometres. Even so, the Auger team can spot these cosmic rays at a rate of only two per month.

The team measured cosmic rays from January 2004 until May 2006, and to ensure a rigorous check on their data, they then looked at a further year's worth of data.

Both sets of data indicated that the most likely source of the rays were nearby active galactic nuclei (AGN), all within 250 million light years of Earth. The rays look to have been deflected by an angle of less than 3.1º by galactic magnetic fields. The researchers' findings are published in Science1

At the heart of AGN is a supermassive black hole, which churns up enough energy to spit out protons with staggering energies of more than 100 x 10 18 eV.

“AGN are very violent situations in space,” says Alan Watson of the University of Leeds, UK, a spokesman for the Pierre Auger Collaboration.

Matching the direction of the rays to these violent galaxies is enough to convince Watson, and the Auger team, that they have found the source of the highest-energy cosmic rays.

Confirmation in sight?

But for some, the statistics aren’t quite good enough to be so certain.

“The Auger collaboration has evidence for correlations pointing to cosmic rays from AGN, but it’s not absolute proof,” says Gordon Thomson of Rutgers University in New York. Thomson is part of the High Resolution (HiRes) Fly’s Eye Collaboration, based at the University of Utah, which does a similar job to the Pierre Auger but on a smaller scale. To get that proof will require other groups to reanalyse their data in light of the Auger claims, Thomson says.

The HiRes team are in the process of preparing a paper that will be submitted within a week or so, outlining exactly that kind of analysis, says Thomson. He remained tight-lipped about their conclusions.

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Shigeru Yoshida of Chiba University in Japan, who is part of Japan's Akeno Giant Air Shower Array project, agrees. “It is important to see if the HiRes observation does confirm this result or not,” he says. If they did, “this result would certainly open an exciting possibility that charged-particle astronomy using the highest-energy cosmic rays becomes an observation science”, he says. 

  • References

    1. The Pierre Auger Collaboration, Science 318, 938-943, (2007) | Article |
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