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 1018More info on IceCube.
Source: IceCube/University of Wisconsin