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   from the issue of November 15, 2007

Snow aids discovery of cosmic ray source


For nearly a century since cosmic rays were first discovered in 1912, scientists have puzzled over their source, particularly of the most powerful cosmic rays.


Now, after more than 3 1/2 years of data collection at the nearly completed Pierre Auger Observatory in western Argentina, a collaboration of 370 scientists and engineers from 17 countries reports it has found that source. In a paper to be published in the Nov. 9 issue of the journal Science, the collaborators report that the highest-energy cosmic rays are produced by relatively uncommon structures known as Active Galactic Nuclei.

Active Galactic Nuclei are thought to be powered by supermassive black holes that are devouring large amounts of matter. They 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 energy and the particles that make up cosmic rays ("cosmic ray" is a misnomer, a term coined before astronomers knew cosmic rays are actually particles like protons and electrons). While most galaxies have black holes at their center, 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.

"This big, international collaboration of scientists came together to make the Pierre Auger Observatory with the purpose of answering three main questions about the origin of the highest-energy cosmic rays," said UNL physicist Greg Snow, one of the collaborators. "This paper quite convincingly answers one of those questions: Where are these highest-energy cosmic rays coming from? We were able to determine the direction of the incoming cosmic rays because of the unique characteristics of the array of detectors we have spread out all over the desert in western Argentina."

The Pierre Auger Observatory, named for a French physicist (1899-1993) who did pioneering work in the study of cosmic rays, is an array of 1,600 radiation detectors spaced 1.5 kilometers (0.9 miles) apart spread over a triangular area of 3,000 square kilometers (1,200 square miles), about the size of Rhode Island. Twenty-four additional inward-looking optical telescopes sit on the periphery of the observatory and measure atmospheric fluorescence produced by extensive showers of charged particles that result from cosmic ray impacts. The scientists needed such a large observatory because the type of high-energy particles they studied hits the Earth at a rate of only about one event per square kilometer per century.

By comparing and combining data from the two sets of detectors, the scientists were able to determine the direction of arrival of the 27 highest-energy events recorded from January 2004 through August 2007, those of more than 57 exa-electron volts.

"We have taken the collection of 27 of our highest-energy cosmic ray events and we have basically taken the direction that we reconstructed and pointed that back into outer space to see if there's something along that line of sight," Snow said. "The very surprising result is 20 out of those 27 point back to a certain type of 'hot' galaxy which has what we call an active galactic nucleus.

"One interesting thing is that the highest-energy cosmic rays are not coming from our galaxy, they're coming from other galaxies, and the particular galaxies our results point back to are rare and among the most interesting objects in the universe. Only one out of a thousand galaxies has this particular property that we classify as having an active galactic nucleus."

Snow said another aspect of the discovery is that all the AGNs in question are within about 200 million light years from Earth, but in the Virgo supercluster of galaxies, which includes the Milky Way. "On the scale of the whole universe, that distance is fairly close to us," he said.

For the seven cosmic ray sources that couldn't be determined, Snow offered two explanations. Some of the sources lie close to the plane of the Milky Way and are obscured by stars and dust in the center of the galaxy, while in other cases, there may be AGNs out there, but they just haven't been confirmed and cataloged yet.

Snow said his group has some preliminary results toward answering the two other central questions about cosmic rays - their distribution between high- and low-energy events, and the precise nature of the particles, whether they're protons, electrons or something else. He said work continues at the Auger Observatory on solving those problems, while a proposal is in the works for a Northern Hemisphere observatory in southeastern Colorado that would give the Pierre Auger Collaboration a view of the entire universe.



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