Cosmic Rays: They Aren’t What We Thought They Were

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The origin of cosmic rays has been one of the most enduring mysteries in physics, and it looks like it’s going to stay that way for a while longer. One of the leading candidates for where cosmic rays come from is gamma ray bursts, and physicists were hoping a huge Antarctic detector called the IceCube Neutrino Observatory would confirm that theory. But observations of over 300 GRB’s turned up no evidence of cosmic rays. In short, cosmic rays aren’t what we thought they were.

But, just like Thomas Edison who said that “every wrong attempt discarded is another step forward,” physicists view this latest finding as progress.

“Although we have not discovered where cosmic rays come from, we have taken a major step towards ruling out one of the leading predictions,” said IceCube principal investigator and University of Wisconsin–Madison physics professor Francis Halzen.

Cosmic rays are electrically charged particles, such as protons, that strike Earth from all directions, with energies up to one hundred million times higher than those created in man-made accelerators. The intense conditions needed to generate such energetic particles have focused physicists’ interest on two potential sources: the massive black holes at the centers of active galaxies and gamma ray bursts (GRBs), flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies.

IceCube is using neutrinos, which are believed to accompany cosmic ray production, to explore these two theories. In a paper published in the April 19 issue of the journal Nature, IceCube scientists describe a search for neutrinos emitted from 300 gamma ray bursts observed, most recently in coincidence with the SWIFT and Fermi satellites, between May 2008 and April 2010. Surprisingly, they found none – a result that contradicts 15 years of predictions and challenges one of the two leading theories for the origin of the highest energy cosmic rays.

The detector searches for high-energy (teraelectronvolt; 10¹²-electronvolt) neutrinos, and in their paper the team said they found an upper limit on the flux of energetic neutrinos associated with GRBs that is at least a factor of 3.7 below the predictions. This implies that either GRBs are not the only sources of cosmic rays with energies greater than 10¹⁸electronvolts, or the efficiency of neutrino production is much lower than has been predicted. Either way, the scientists say, our current theories of cosmic-ray and neutrino production in GRBs will need to be revisited. "The result of this neutrino search is significant because for the first time we have an instrument with sufficient sensitivity to open a new window on cosmic ray production and the interior processes of GRBs," said IceCube spokesperson and University of Maryland physics professor Greg Sullivan. "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." IceCube is a particle detector at the South Pole that records the interactions of a nearly massless neutrinos. The instruments observe neutrinos by detecting the faint blue light produced in neutrino interactions in ice. Neutrinos can easily travel through people, walls, or entire planets, like Earth. In order to detect their rare interactions, IceCube is built on an enormous scale. One cubic kilometer of glacial ice, enough to fit the great pyramid of Giza 400 times, is instrumented with 5,160 optical sensors embedded up to 2.5 kilometers deep in the 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. Scientists say that improved theoretical understanding and more data from the compete IceCube detector will help scientists better understand the mystery of cosmic ray production. IceCube is currently collecting more data with the finalized, better calibrated, and better understood detector. IceCube is operated by a collaboration of 250 physicists and engineers from the USA, Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia and Barbados. More info on IceCube.

Paper in Nature.

Source: IceCube/University of Wisconsin