Cosmic rays are charged subatomic particles that streak to Earth from deep in outer space. A few rare cosmic rays are extraordinarily powerful, with energies up to 100 million times greater than any attained by human-made particle colliders, such as CERN's Large Hadron Collider. The sources of these cosmic rays are a mystery.
"Nature is capable of accelerating elementary particles to macroscopic energies," said study co-author Francis Halzen at the University of Wisconsin-Madison, principal investigator at the IceCube Neutrino Observatory, a massive telescope designed to find the tiny subatomic particles. "There are basically only two ideas on how she does this — in gravitationally driven particle flows near the supermassive black holes at the centers of active galaxies, and in the collapse of stars to a black hole, seen by astronomers as gamma-ray bursts."
The prime suspect
Gamma-ray bursts are the most powerful explosions in the universe. They can emit as much energy as our sun during its entire 10-billion-year lifetime in anywhere from milliseconds to minutes.
"Some gamma-ray bursts are thought to be collapses of supermassive stars — hypernovas — while others are thought to be collisions of black holes with other black holes or neutron stars," said study co-author Spencer Klein of the U.S. Department of Energy's Lawrence Berkeley National Laboratory. "Both types produce brief but intense blasts of radiation."
New evidence may now rule out gamma-ray bursts as sources of these ultra-high-energy cosmic rays.
Researchers employed the IceCube neutrino detector, an array of thousands of detectors encompassing a cubic kilometer of clear Antarctic ice at the South Pole. Neutrinos are ghostly particles that often pass right through matter, only rarely striking atoms.
"This is a coming-of-age for neutrino astronomy — the first time we're able to use neutrino data as a new way of looking at astrophysical objects and say something substantive about them," said study co-author Nathan Whitehorn, a physicist at the University of Wisconsin-Madison, who led the recent gamma-ray burst research with Peter Redl of the University of Maryland.
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