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Media contact: Steve Carr, (505) 277-1821
scarr@unm.edu

November 8, 2007

Auger Observatory Closes in on Long-Standing Mystery; Links Highest-Energy Cosmic Rays with Violent Black Holes
UNM scientists play active role in research

Scientists of the Pierre Auger Collaboration, which includes researchers in the University of New Mexico’s Physics and Astronomy Department, announced today that Active Galactic Nuclei or AGNs, are the most likely candidate for the source of the highest-energy cosmic rays that hit Earth. The results will appear in the Nov. 9 issue of the journal Science.

Using the Pierre Auger Observatory in Argentina, the largest cosmic-ray observatory in the world, a team of scientists from 17 countries 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 centers.

Supermassive black holes that are devouring large amounts of matter are thought to power AGNs. 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 particles and energy.

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.

“We looked at our cosmic rays and tried to see if they correlated with a category of astrophysical sources [the AGNs] because there aren’t enough of them to come from any one source,” said UNM Professor John Matthews. “Normally you would say it should be some bright object in the sky. The trouble is they aren’t. We’re stuck trying to correlate what we actually measure with possible sources. The AGNs are supermassive black holes and that’s what’s cool about them.”

“There are very few events to measure to help make the correlation,” said graduate student J. Doug Hague, another researcher from UNM. “Imagine your telescope only gives you one photon a month. How would you determine where the stars are? Here it takes a year or more with one single light pulse [one cosmic ray] at a time.”

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 our 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.

“It turns out that supermassive black holes tend to be everywhere and they are incredibly efficient at converting gravitational energy into other forms of particle beams,” said Matthews.

Other UNM scientists participating in the research include Professor Michael Gold, Instrumentation Specialist Bill Miller and Post Doctorate Bernie Becker.

This particular field of study at UNM was initiated by Physics and Astronomy Professor John Linsley in the early-19 60s when he set up an experiment on the West Mesa in Albuquerque. His concept of placing detectors spread out over a certain area is similar to the method used by the Pierre Auger group which captured records of cosmic ray showers through an array of 1,600 particle detectors placed 1.5 kilometers (approximately one mile) apart in a grid spread across 3,000 square kilometers (1,200 square miles).

In the Auger experiment , 24 specially designed telescopes [also] 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.

“We looked at one set of data for a two year period from 2004-06,” said Becker, who also enlisted the assistance of Tim Thomas at the UNM Supercomputing Center to examine the data. “Then we looked at another [independent] set of data from that date on forward and found that it repeats. The conclusion was that there was a one percent probability of that repeating at random.“

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. Auger scientists so far have recorded 77 cosmic rays with energy above 4 x10 19 electron volts, or 40 EeV. This is the largest number of cosmic rays with energy above 40 EeV recorded by any observatory. At these ultra-high energies, the uncertainty in the direction from which the cosmic ray arrived is less than one degree, allowing scientists to determine the location of the particle’s cosmic source.

“The big open question is how are these extremely high energy cosmic rays produced,” said Gold. “This [result] gives us a big clue with a class of objects that seem to be the source. When we combine modeling of this type of source with measurements of the energy spectrum, hopefully it will give us more information to find out exactly what the mechanism is.”

“We took the obvious catalog of objects, mainly nearby supermassive black holes and asked if our cosmic rays seem to be from the same region of space? Miraculously, the answer was yes,” said Matthews. “I’d say we’ve had a fantastically good time doing the research.”

Scientists think that most galaxies have black holes at their centers, with masses ranging from a million to a few billion times the mass of our sun. The black hole at the center of our Milky Way galaxy weighs about three million solar masses, but it is not an AGN.

Galaxies that have an AGN seem to be those that suffered a collision with another galaxy or some other massive disruption in the last few hundred million years. The AGN swallows the mass coming its way while releasing prodigious amounts of radiation. The Auger result indicates that AGNs may also produce the universe's highest-energy particles.

The Auger collaboration discovered that the 27 highest-energy events, about one per month over two years, with energy above 57 EeV, do not come equally from all directions. Comparing the clustering of these events with the known locations of 318 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.

“When I started working on the project, John had been designing a system for calibrating the [Auger] telescopes,” Miller said. “So we’ve been designing and installing many little pieces over the years [to assemble the complete] detector. In addition to the physics analysis, you actually have to build the experiment. Now that part kind of takes a backseat as it functions flawlessly as I understand.”

A team of more than 370 scientists and engineers from 17 countries is building the Pierre Auger Observatory. The collaboration is truly international with no country contributing more than 25 percent of the $54 million cost of construction. The Department of Energy and the National Science Foundation have provided funding in the United States for the project.

Groundbreaking for the southern hemisphere site of the Pierre Auger Observatory took place on March 17, 1999, in Argentina’s Mendoza Province. Following a period of detector deployment and testing, scientific data collection began in January 2004.

Because of the observatory’s size, which is similar to the state of Rhode Island, the Auger Observatory can record about 30 ultra-high-energy events per year. 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.

Photos and background information:
http://www.auger.org/media

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