Scientists Weed Out 15-Year-Old Theory On Cosmic Rays
April 19, 2012

Scientists Weed Out 15-Year-Old Theory On Cosmic Rays

Researchers using the IceCube Neutrino Observatory have taken the first steps to determine how the highest energy cosmic rays are produced.

Francis Halzen, the IceCube principal investigator, said that although they haven't discovered where cosmic rays have come from, they have made a big leap forward in ruling out one of the leading predictions.

The researchers wrote in a paper published in the journal Nature that they searched for neutrinos emitted from 300 gamma ray bursts between May 2008 and April 2010.

During their study, they found no neutrinos, which is 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 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," IceCube spokesperson and University of Maryland physics professor Greg Sullivan said in a press release.

He said the absence of neutrinos from gamma ray bursts (GRB) has made scientists re-evaluate 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 IceCube Neutrino Observatory is a neutrino telescope at the geographical South Pole in Antarctica, and is operated by physicists and engineers in several countries throughout the world.

The observatory looks for neutrinos by detecting the faint blue light produced in neutrino interactions in ice.

Neutrinos can easily travel through people, walls, or Earth, so in order to detect their interactions, IceCube is built on a large scale.

Gamma ray bursts are usually first observed by satellites using X-rays, and they are seen about once per day.  GRBs are so bright that they can be seen from halfway across the visible universe.

The explosions generally last for just a few seconds, which is enough time for them to outshine everything else in the universe.

The fireball model predicted that when the expected flux from all the samples had been calculated, at least 8.4 related muon events would be found within 10 degrees of a GRB during the seconds or minutes when it was at its peak.

"According to a leading model, we would have expected to see 8.4 events corresponding to GRB production of neutrinos in the IceCube data used for this search.  We didn't see any, which indicates that GRBs are not the source of ultra-high-energy cosmic rays," Spencer Klein of the U.S. Department of Energy's Lawrence Berkeley National Laboratory, said in a press release.

Nathan Whitehorn from the University of Wisconsin-Madison, who led the recent GRB research with Peter Redl of the University of Maryland, said that this result represents a "coming-of-age of neutrino astronomy."

"IceCube, while still under construction, was able to rule out 15 years of predictions and has begun to challenge one of only two major possibilities for the origin of the highest-energy cosmic rays, namely gamma-ray bursts and active galactic nuclei," Whitehorn said in a press release.

Scientists will be able to help better understand the mystery of cosmic ray production through improved theoretical understanding and more data from the complete IceCube detector.

"While not finding a neutrino signal originating from GRBs was disappointing, this is the first neutrino astronomy result that is able to strongly constrain extra-galactic astrophysics models, and therefore marks the beginning of an exciting new era of neutrino astronomy," Redl said in the press release.


Image Caption: IceCube´s 5,160 digital optical modules are suspended from 86 strings reaching a mile and a half below the surface at the South Pole. Each sphere contains a photomultiplier tube and electronics to capture the faint flashes of muons speeding through the ice, their direction and energy — and thus that of the neutrinos that created them — tracked by multiple detections. At lower left is the processed signal of an energetic muon moving upward through the array, created by a neutrino that traveled all the way through the Earth.