Cosmic rays not from gamma bursts

THE UNIVERSITY OF ADELAIDE

Share on print Cosmic rays are electrically charged particles, such as protons, that strike Earth from all directions. Image: goktugg/iStockphoto An international study involving a University of Adelaide researcher has produced surprising results about one of the most enduring mysteries in physics - the origin of cosmic rays. First discovered 100 years ago, cosmic rays are electrically charged particles, such as protons, that strike Earth from all directions, with energies up to 100 million times higher than those created in man-made accelerators. Physicists have focused their interest on two potential sources: the massive black holes at the centre of active galaxies, and the exploding fireballs observed by astronomers as gamma ray bursts. The IceCube Neutrino Observatory, a massive detector in Antarctica, is exploring these theories by studying neutrinos, which are believed to accompany cosmic ray production. Dr Gary Hill, ARC Future Fellow with the University of Adelaide's School of Chemistry & Physics, is a member of the IceCube collaboration. He has spent seven Antarctic summers working on the IceCube observatory and has lived and worked in Antarctica 12 times in the past 15 years. In a paper published today in the journal Nature, the IceCube collaboration describes a search for neutrinos emitted from 300 gamma ray bursts (GRBs) observed between May 2008 and April 2010. Surprisingly, they have found no evidence of neutrinos - a result that contradicts 15 years of predictions. "This is the most important result so far from the IceCube observatory," says Dr Hill. "Gamma ray bursts don't seem to make neutrinos as we previously thought, which means they probably aren't making cosmic rays either. "This result has implications for other experiments around the world, including experiments like the Auger cosmic ray observatory that researchers in Adelaide are involved in, and will help to focus the search for the origin of cosmic rays even further." IceCube spokesperson and University of Maryland physics professor Greg Sullivan says the IceCube observatory is the first instrument "with sufficient sensitivity to open a new window on cosmic ray production and the interior processes of GRBs". "The unexpected absence of neutrinos from GRBs has forced a re-evaluation of the theory for production of cosmic rays and neutrinos in a GRB fireball and possibly the theory that high-energy cosmic rays are generated in fireballs," Professor Sullivan says. Principal investigator and University of Wisconsin-Madison physics professor Francis Halzen says: "Although we have not discovered where cosmic rays come from, we have taken a major step towards ruling out one of the leading predictions." Background - about IceCube and gamma ray bursts Completed in December 2010, IceCube is a high-energy neutrino telescope at the geographical South Pole in Antarctica, operated by a collaboration of 250 physicists and engineers from the USA, Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia and Barbados. IceCube observes neutrinos by detecting the faint blue light produced in neutrino interactions in ice. Neutrinos are of a ghostly nature; they can easily travel through people, walls, or the planet Earth. To detect their interactions, IceCube is built on an enormous scale: 5160 optical sensors embedded up to 2.5 kilometres deep in the ice, spanning one cubic kilometre of glacial ice. GRBs, the universe's most powerful explosions, are usually first observed by satellites using X-rays and/or gamma rays. GRBs are seen about once per day, and are so bright that they can be seen from half way across the visible Universe. The explosions usually last only a few seconds, and during this brief time they can outshine everything else in the universe. Improved understanding and more data from the complete IceCube detector will help scientists better understand the mystery of cosmic ray production. Editor's Note: Original news release can be found here.