A $272m telescope buried deep beneath the South Pole has failed to find any neutrinos accompanying exploding fireballs in space, undermining a leading theory on how cosmic rays are born.
IceCube – a detector consisting of 5,160 optical sensors embedded up to 2.5km deep in the ice – searched for evidence of neutrinos emitted from 300 fireballs known as gamma ray bursts (GRB) observed between May 2008 and April 2010. Understanding the origin of cosmic rays would provide insight into the most energetic phenomena in the Universe.
Researchers said the search did not find a single neutrino – which would have appeared as a flash of blue light beneath the surface of the ice. The results – published in Nature – challenge the idea the cosmic rays originate from gamma ray bursts.
“Although this is a negative result it illustrates that cosmic neutrino detection has come of age – IceCube has achieved the necessary sensitivity to neutrino fluxes expected from likely sources of cosmic rays,” said Professor Subir Sarkar who leads the UK involvement in the experiment. “It should not be long now before we find the real sources.”
The findings implies GRBs are not the only sources of cosmic rays with energies exceeding 1018 electronvolts and is likely to focus attention on the other prime candidate for creating cosmic rays – supermassive black holes found at the centre of active galaxies.
“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 Professor Greg Sullivan, IceCube spokesperson.
“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.”
The fireball theory predicts that very energetic astrophysical events – gamma ray bursts – are required to accelerate cosmic rays to above 1018 electronvolts. This acceleration should be accompanied by neutrinos, which are produced by the decay of charged pions created in interactions between high energy cosmic ray protons and gamma rays.
“Although we have not discovered where cosmic rays come from, we have taken a major step forward ruling out one of the leading predictions,” said Francis Halzen, IceCube principal investigator.