An international team of scientists said Thursday that they have tracked down the origin of the mysterious "Oh-My-God" particle -- a cosmic ray bearing energies millions of times larger than the most powerful particle accelerator can produce on Earth.
Researchers at the Pierre Auger Observatory, a complex of detectors spread over a Rhode Island-sized slice of the South American prairie, said the most likely source for these ultra-high-energy particles is a type of black hole found at the center of some galaxies.
These violent phenomena are called active galactic nuclei because they both gobble up energy from the surrounding space and spit some out with tremendous force.
"This is a fundamental discovery. We have taken a big step forward in solving the mystery of the nature and origin of the highest-energy cosmic rays," said Nobel laureate James Cronin, a professor emeritus at the University of Chicago and coauthor of the paper published in the current issue of the journal Science.
Cosmic rays are charged particles that can strike with the force of a thrown baseball. Fortunately for humans, Earth's atmosphere serves as a shield, preventing the particles from reaching the surface.
The origin of cosmic rays, particularly the high-energy ones, has been a mystery since their discovery in 1938 by French scientist Pierre Auger. Scientists have only been able to speculate over what tumultuous processes could accelerate particles to energy levels 100 million times more powerful than anything produced at Fermi National Accelerator Laboratory in Illinois, Earth's most powerful accelerator.
One of the chief candidates was black holes at the center of many galaxies, mainly because these are some of the most energetic objects in the universe.
But tracking the source of the particles has been difficult, in part because of their rarity. Each century, only one of the highest-energy particles -- sometimes called Oh-My-God particles -- strikes the atmosphere over any particular spot on Earth.
To research them, "You either need very long-lived scientists or an instrument covering a very large area," said Henry Glass, a Fermilab particle physicist and a coauthor of the paper.
Besides being large, the detector had to be sophisticated enough to spot the cascade of billions of secondary particles that rain down when a high-energy cosmic ray hits the upper atmosphere.
That was the thinking behind the $54-million Auger Observatory, a collaboration of 17 nations and 370 scientists. The facility covers 1,200 square miles of Argentina's Pampa Amarilla, or yellow prairie.
To trace the course of the secondary particles, known as extensive air showers, the observatory employs two types of detectors. One is made up of scores of ultra-pure-water tanks that register the energy and path of arriving secondary particles. The second is a set of telescopes spread around the observatory to spot the ultraviolet fluorescence that occurs when a high-energy particle hits the nitrogen in the upper atmosphere.
"What we're doing is using the atmosphere as an amplifier," Glass said.
The Auger detectors went to work nearly four years ago, even before construction was complete.
So far, Glass said, the detectors have tracked 27 high-energy cosmic ray events. At least 20 led back to previously known active galactic nuclei.
Two of the high-energy events were traced to a single galaxy, Centaurus A -- also known as the Hamburger Galaxy -- about 13.7 million light-years away.
Active galactic nuclei make up only about 1% of all galactic black holes. New ones are made when galaxies collide. The black holes spring to life, gobbling up pieces of the other galaxy.
Tracking the high-energy rays is just the first stage in the new field of cosmic ray astronomy, said Cronin, who conceived the Auger Observatory.
"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," he said.
Scientists also hope to open a similar observatory in the Northern Hemisphere. A site in Colorado has already been selected, Glass said.
"We are opening a new window in astronomy," said Joao de Mello Neto, an astronomer at the University of Rio de Janeiro in Brazil and a coauthor of the paper.