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Discovery links highest-energy cosmic rays with violent black holes

on November 08, 2007 12:00 AM
Discovery links highest-energy cosmic rays with violent black holes
Thursday, November 8, 2007

Penn State scientists and other members of the 17-nation Pierre Auger Collaboration will announce Nov. 9 their discovery that Active Galactic Nuclei -- thought to be powered by supermassive black holes -- are the most likely source of the highest-energy cosmic rays that hit Earth.  Using the Pierre Auger Observatory in Argentina, the largest cosmic-ray observatory in the world, the research team has closed in on a long-standing scientific mystery by linking the sources of the origins of these mysterious highest-energy particles to the locations of nearby galaxies that have active nuclei in their centers. The results will appear in the Nov. 9 issue of the journal Science.

"This discovery not only reveals the likely origins of the highest-energy particles in the universe, but it also reveals, for the first time, that those particles are almost surely protons," said Paul Sommers, professor of physics at Penn State and associate director of the Penn State Institute for Gravitation and the Cosmos. "With this knowledge, it now becomes possible to study the properties of particle interactions at these energies, which are much higher than can be attained using particle accelerators on Earth."

Sommers has worked on the Auger Project since its inception in 1992 and was the task leader for its air fluorescence detectors during the design-and-development stages of the project. He subsequently served as the co-chair of the Auger Project's international Collaboration Board.

Most galaxies have supermassive black holes at their centers, but only a fraction of them are Active Galactic Nuclei (AGNs), long considered sites where high-energy particle production might take place. These AGNs swallow gas, dust and other matter from their host galaxies and spew out particles and energy. 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," said 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 protons and atomic nuclei that travel across the universe at close to the speed of light. When these particles smash into the upper atmosphere of the planet, they create a cascade of secondary particles called an air shower that can spread across 40 or more square kilometers (15 square miles) as they reach the Earth's surface.

"This result heralds a new window to the nearby universe and the beginning of cosmic-ray astronomy," said Watson, a spokesperson of the Pierre Auger Collaboration. "As we collect more and more data, we may look at individual galaxies in a detailed and completely new way. As we had anticipated, our observatory is producing a new image of the universe based on cosmic rays instead of on light."

The Pierre Auger Observatory records cosmic-ray showers through an array of 1,600 particle detectors placed 1.5 kilometers (about one mile) apart in a grid spread across 3,000 square kilometers (1,200 square miles). 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.

"It's difficult to truly comprehend the scale of the effort required to investigate these extremely rare and most energetic of particles," said Stephane Coutu an associate professor of physics and astronomy and astrophysics at Penn State and a member of the Auger team since 1997. "The detectors are deployed out on the Argentine pampa nearly a mile apart, over an area the size of Rhode Island, so that one can seldom even see more than two or three of the detectors with the naked eye. Yet dozens of detectors can record particles of an air shower produced by a single nergetic cosmic ray. It really is a sobering experience being out there and realizing that such high-energy particles exist in nature."

To aid in visualizing the layout of the Auger Observatory, the Penn State team has put together freely downloadable movies and an interactively explorable 3-D model of the observatory structures and detectors using Google Earth. The movie in English is at online. The model files, other versions of the movie and more information sources are at online, along with instructions on installing and using the model.

"Spending a few minutes navigating around the Argentine site, tilting the field of view and flying over the detector array in Google Earth, really brings home the scale of the task we have undertaken," Coutu commented. "Seeing the first hints of the astrophysical origins of these extraordinarily energetic particles is a welcome reward after the many years of effort it took to design and deploy this largest of observatories."

The Auger collaboration discovered that the 27 highest-energy events, with energy above 57 EeV, do not come equally from all directions. Comparing the clustering of these events with the known locations of 381 AGNs, the collaboration found that most of these events correlated well with the locations of AGNs in some nearby galaxies, such as Centaurus A.

While the observatory has recorded almost a million cosmic-ray showers, only the rare, highest-energy cosmic rays can be linked to their sources with sufficient precision because they are deflected less by magnetic fields. Unfortunately, such energetic cosmic rays hit Earth at a rate of only about one per square kilometer per century -- requiring a very large observatory.

The Auger collaboration is developing plans for a second, larger installation in Colorado to extend coverage to the entire sky while substantially increasing the number of high-energy events recorded.

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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