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IceCube chills cosmic ray theories

IceCube, an ice-bound telescope that looks for neutrinos rather than photons, has cast doubt on the long-held assumption that gamma-ray bursts are responsible for the highest-energy cosmic rays that rocket around the Universe.

Gamma-ray bursts (GRBs), short-lived jets of light that mark the collapse of stars and the birth of black holes, were one of only two mechanisms thought to be capable of generating the cosmic rays, particles with energies higher than 10^18 electronvolts. In conjunction with the high-energy cosmic rays, the GRBs should also produce a flux of high-energy neutrinos created during a decay process. And since the nearly massless neutrinos are also chargeless, they are not bent by galactic magnetic fields en route to Earth — making them a better tracer of the original source object. But IceCube, which can detect these neutrinos as they barrel through the Earth to a set of sensors frozen in Antarctic ice, did not see the expected flux associated with several hundred GRBs in the northern sky. A paper published today in Nature describes the results.

“My conclusion from this paper is that GRBs don’t make the cosmic rays,” says IceCube principal investigator Francis Halzen, of the University of Wisconsin in Madison.

That puts the pressure on other cosmic ray experiments, such as Pierre Auger Observatory in Argentina, which have been unable to confirm hints of a connection between the particles and the other assumed source: active galactic nuclei, the super-massive black holes at the centers of some galaxies that can accelerate particles as they gobble up surrounding material.

The IceCube analysis was based on several hundred GRBs identified by other satellites. The data were collected up to May 2010, while the array was still under construction. The last of 86 strings (each with 60 sensors) was dropped into the ice in December 2010. Halzen says that, in the coming month, the IceCube team will unblind its first dataset from the full array.

Image credit: Danielle Vevea/NSF & Jamie Yang/NSF

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