For Immediate Release
April 18, 2012
Contacts: Lee Tune, 301 405 4679 or ltune@umd.edu

New Findings by IceCube Neutrino Observatory - Gamma Ray Bursts Not Source of Mysterious Cosmic Rays

	Little is know about the ultra high-energy cosmic rays that regularly penetrate the Earth. (Credit: NSF/J.) Click on Image for higher resolution.

COLLEGE PARK, Md. -- Cosmic rays -- high energy particles that bombard the Earth from beyond our solar system -- were discovered 100 years ago. However, their origin remains one of the most enduring mysteries in physics. Researchers at the University of Maryland and other institutions are using the IceCube Neutrino Observatory, a massive detector in Antarctica, to hone in on how the highest energy cosmic rays are produced.

In a study in the April 19 issue of the journal Nature (published online today), the IceCube collaboration says that a search for neutrinos emitted from 300 gamma ray bursts, unexpectedly found none! Because neutrinos are believed to accompany cosmic ray production, this result 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 [gamma ray bursts]," 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."

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 the exploding fireballs observed by astronomers as gamma ray bursts (GRBs).

IceCube Lab in the moonlight in Antarctica (Credit: NSF/S. Lidstrom) Click on Image for higher resolution.

IceCube is using neutrinos to explore these theories. The paper in Nature, describes a search for neutrinos emitted from 300 gamma ray bursts that were observed in coincidence with the SWIFT and Fermi satellites, between May 2008 and April 2010.

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 principal investigator and University of Wisconsin - Madison physics Professor Francis Halzen.

IceCube observes neutrinos by detecting the faint blue light produced in neutrino interactions in ice. Neutrinos are of a ghostly nature; they can easily travel through people, walls, or the planet Earth. To compensate for the antisocial nature of neutrinos and 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.

Illustration of a gamma ray burst. (Credit: NASA/D.Berry)) Click on Image for higher resolution.

IceCube & UMD

IceCube is a high energy neutrino telescope at the geographical South Pole in Antarctica, funded by the National Science Foundation (NSF) and operated by a collaboration of 250 physicists and engineers from the USA, Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia and Barbados. Construction of IceCube was finished in December 2010.

Collaboration spokesman Sullivan leads the University of Maryland contingent, which also includes UMD physics professors Kara Hoffman and Jordan Goodman, research scientist Erik Blaufuss, post-doctoral researchers Alex Olivas and Henrike Wissing and several UMD graduate students, including recent graduates Peter Redl and Kevin Meagher, whose analyses provided major contributions to the Nature paper. Blaufuss is the leader of the gamma ray burst analysis working group inside the collaboration, and UMD is one of the lead institutions in this analysis.

"Our group has been a member of IceCube for a decade now, helping to design and build the detector at the South Pole, Antarctica," says Blaufuss. "Our primary responsibilities are in the computing and software infrastructure to support the experiment: software for filtering data in real-time at the South Pole, and the software tools used to analyze data throughout the entire collaboration."

Improved theoretical understanding and more data from the complete 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.

UMD's Hoffman is now spearheading the next generation of ultra-high-energy neutrino detectors now being built at the South Pole. The Askaryan Radio Array will detect neutrinos produced by the
interaction of the highest energy cosmic rays with the cosmic microwave background radiation which permeates the universe by observing the radio frequency signal produced by the neutrino interactions in the South Pole glacial ice. Instrumenting the ice with radio antennas allows for the construction of an very large detector, envisioned to be 100 square kilometers in area, that is capable of detecting hundreds of these rare interactions. Funded by the National Science Foundation, the project is currently in the prototype phase.

UMD Science Contacts:
Professor Greg Sullivan
sullivan@icecube.umd.edu
301-405-6035

Dr. Erik Blaufuss
blaufuss@icecube.umd.edu
301-405-6077

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