Observations put theory of the origin of cosmic radiation to the test
Hamburg, 18 April 2012. The most powerful particle accelerators are found in space: some subatomic particles that rain down from space on earth's atmosphere have energies up to one hundred million times higher than those created in the Large Hadron Collider (LHC), the most powerful accelerator on earth. However, it is still quite mysterious how these so-called cosmic rays are accelerated to such high energies. With the world's largest neutrino telescope IceCube in Antarctica, scientists have investigated one of the possible types of cosmic superaccelerators and discovered that they are probably not the main source of the highest energy cosmic rays. This result calls for a re-evaluation of one of the two leading hypothesis on the origin of extremely energetic cosmic particles, the international team of scientists lead by Nathan Whitehorn from the University of Wisconsin (USA) reports in the scientific journal "Nature". The IceCube collaboration comprises some 40 institutes from ten countries, among them eight German universities and the German accelerator centre DESY.
Cosmic radiation, which was discovered a hundred years ago, is like a constant particle shower from outer space. Some hydrogen nuclei (protons) that form a major part of this radiation have as much energy as a powerfully-hit tennis ball, although the diameter of a tennis ball is 40 trillion times larger. "We know that this high-energetic cosmic radiation exists, but we don’t know where it comes from," emphasises DESY scientist Alexander Kappes, who uses the neutrino telescope to track down the origin of cosmic radiation. The particles of cosmic radiation are electrically charged and, on their journey through space, are deflected by numerous magnetic fields. Therefore, it is no longer possible to determine their actual source from the direction they come from when they hit the earth.
Promising candidates for the sources of highest-energy particles are massive black holes at the centres of active galaxies and gamma ray bursts (GRBs). "Apart from the Big Bang, gamma ray bursts are the most powerful explosions we know of in the universe," says Kappes, who is also a visiting professor at Berlin's Humboldt University. For a few seconds they can outshine everything else in the universe in the range of gamma radiation. Scientists think that long gamma ray bursts that last more than two seconds are the collapse of the core of a very massive star in a distant galaxy, which produces a black hole.
This process generates enough energy to accelerate the subatomic particles of cosmic radiation to the energies observed. However, neutrinos should be produced alongside these high-energy atomic nuclei. These ghostly particles are ultra-light cousins of the electron that travel through almost everything unhindered; this is why extremely large detectors are needed to see them. IceCube is the world’s most sensitive neutrino telescope and uses the eternal ice of the South Pole as part of the detector. Beneath the ice’s surface, with more than 5000 individual sensors (photomultipliers) deployed in about a cubic kilometre of Antarctic ice, IceCube tracks down the extremely rare collisions of a neutrino with an atomic nucleus.
The international IceCube team investigated around 300 gamma ray bursts from the period of 2008 to 2010. If gamma ray bursts are the sources of high-energy cosmic particle radiation, the neutrinos produced alongside should reach the earth directly, as they are electrically neutral and thus not deflected by magnetic fields. “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 IceCube spokesperson and University of Maryland physics professor Greg Sullivan.
However, within the investigation period of two years, IceCube has surprisingly not found a single neutrino that corresponds to one of the about 300 bursts explored. "Two possible explanations can be derived from this observation," said Kappes. "It is possible that the postulation that gamma ray bursts are the main source of the extremely high-energy cosmic radiation is wrong. The other explanation is that our calculation models of the processes in gamma ray bursts are based on incorrect or highly simplified assumptions." In any case, the current models of cosmic radiation and neutrino production in gamma ray bursts have to be reworked.
"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. In the coming years, with the complete upgrade and with increasing data taking time, IceCube will provide important information to clarify these issues.
IceCube is a high-energy neutrino telescope at the geographical South Pole in Antarctica. 5,160 optical sensors (photomultiplier) embedded up to 2.5 kilometers deep in the ice, search for signals of rare neutrino interactions in the ice. The facility has a total volume of one cubic kilometre, enough to fit the great pyramid of Giza 400 times. The world's largest and most sensitive neutrino telescope is operated by a collaboration of 250 physicists and engineers from the USA, Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia and Barbados. Participating universities and institutes from Germany are Rheinisch-Westfälische Technische Hochschule Aachen, Humboldt-Universität zu Berlin, Ruhr-Universität Bochum, Rheinische Friedrich-Wilhelms-Universität Bonn, Technische Universität Dortmund, Johannes-Gutenberg-Universität Mainz, Technische Universität München, Bergische Universität Wuppertal and the accelerator centre Deutsches Elektronen-Synchrotron DESY. The internationally organized and funded construction was finished in 2010. From Germany, construction and data analysis are supported by the Federal Ministry of Education and Research (BMBF), the German Research Foundation (DFG) and the Helmholtz Alliance for Astroparticle Physics (HAP).
"An Absence of Neutrinos Associated with Cosmic Ray Acceleration in Gamma-Ray Bursts"; Abbasi et al.; "Nature" 2012; Bd. 484, S. 351, DOI: 10.1038/nature11068
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