Nearly 70 years ago, French scientist Pierre Victor Auger (1899-1993) was the first to observe a phenomenon known as an "air shower," a broad cascade of particles occurring in the Earth's upper atmosphere.
Auger speculated that the air showers, which can cover 15 square miles, were caused by particularly powerful cosmic rays. But at the time, he had no mechanism for studying the phenomenon in more detail.
On Thursday, an international team of 370 scientists announced that they have identified the most likely source of the rare particles.
Most Energetic Particles in Nature
On the basis of data collected by the Pierre Auger Observatory in Argentina, scientists now believe that they originate in galaxies containing Active Galactic Nuclei -- particularly massive black holes that are actively converting stellar material into powerful bursts of energy, including high-energy cosmic rays.
Fermilab scientist Paul Mantsch, the project manager for the Collaboration, said in an e-mail interview that the goal of the research is to understand the violent physics processes that propel these cosmic rays to such enormous energy.
"We know how to accelerate particles at accelerators like the Tevatron at Fermilab or the LHC at CERN," Mantsch continued. "But there is as yet no known mechanism for accelerating particles to energies a hundred million of times those of the protons at Fermilab. These are the most energetic particles in nature. We have made the first step in closing in on this mystery."
Cosmic rays -- protons and atomic nuclei stripped of their enveloping cloud of electrons -- routinely strike the Earth's atmosphere. The majority are low- and medium-energy cosmic rays that are emitted by our sun and other nearby stars. As they travel towards Earth, the paths of such cosmic rays are bent and twisted by the magnetic fields of the objects they pass, making it impossible to determine their source. Until yesterday's announcement, the only specifically identified source of cosmic rays was the sun.
An Elusive Quarry
A tiny percentage of cosmic rays, however, are powerful enough to travel through space with relatively little interference. Cosmic rays with that much energy are quite rare -- striking each square kilometer of earth on average just once a century.
To make the study of high energy cosmic rays practical, the Pierre Auger Collaboration (a coalition of seventeen nations) built a widespread observatory in Argentina, consisting of 1,400 particle detectors spread across 1,200 square miles (roughly the size of Rhode Island). Because the facility began operating in 2004, it has detected 77 high energy cosmic rays, and scientists were able to calculate with a fair degree of certainty the region of the sky from which the particles originated.
Cosmic ray scientists are eager to expand upon their work. Another 200 detectors are planned for Argentina, and the Collaboration is drawing up plans for an even larger facility, dubbed "Auger North," in the United States.
"The Auger Observatory site near Lamar, Colorado will allow us to scan the whole sky for sources," Mantsch said. "We also want the northern site to be several times bigger than the southern site so that we can accumulate enough events to effectively study the sources. This has to, of course, be consistent with reasonable cost."
By the standards of most space-related projects, the Auger Collaboration has been a bargain -- just $54 million so far, with no nation paying more than 25 percent.