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Posted: Mar 04, 2016

High-energy cosmic rays: Galactic or extragalactic?

(Nanowerk News) It appears that radio-wave measurements have discovered a new component of the Galactic cosmic ray population. An international research group has used the sensor array LOFAR (LOw Frequency ARray) to measure the properties of cosmic rays, and has found a signal component coming from our galaxy within the energy range from 1017 to 1017.5 electron volts (eV) that was previously unknown (Nature, "Radio detections of cosmic rays reveal a strong light mass component at 1017 - 1017.5 eV").
LOFAR-Station Potsdam-Bornim
LOFAR-Station Potsdam-Bornim.
The use of radio-frequency measurements has developed into a new and important tool in the study of the origins of cosmic rays. For extremely high-energy cosmic ray particles, energies below 1018 eV are believed to originate within our own galaxy, the Milky Way, whereas particles with greater energies are thought to originate in other galaxies or quasars in the universe. In the range of radio-frequencies the Milky way is one of the brightest objects in the sky.
LOFAR is a Dutch radio telescope with partners in Germany, France, Sweden, Poland, and the UK. LOFAR is made up of thousands of sensors located at numerous LOFAR stations, one of which is located in Potsdam-Bornim. The LOFAR measurements of the mass distribution of cosmic rays provides information on their probable origin, whether this energy range is dominated by Galactic (from our Milky Way) or extragalactic cosmic rays. In essence, high mass particles (iron nuclei and heavier in the table of elements) should stem from our Milky Way, but only low mass particles like protons (i.e. hydrogen nuclei) are expected from other galaxies.
At least that is what the models predict. The LOFAR measurements, however, find, unexpectedly, both protons and helium nuclei within the energy range from 1017 to 1017.5 electron volts (eV).
"The new results allow us to conclude that within this energy range, either there are additional cosmic rays from within our Milky Way whose origin is not yet clear, or that the extragalactic component extends further to lower energies that were previously thought to belong exclusively to cosmic rays from our Milky Way", says co-author James Anderson of the GFZ German Research Centre for Geosciences.
The results are interesting for many research areas of the GFZ. Radio telescopes are important instruments for geodesy for precise position measurements and orientation parameters of the Earth in space. Cosmic rays produce specific isotopes, radionuclides, in the atmosphere and on the surface of the Earth that are used to determine the ages of stones and glaciers. And even the Earth's magnetic field and space weather are influenced by cosmic rays. James M. Anderson is a scientist in the VLBI (very long baseline interferometry) Group of GFZ-Section "Space Geodetic Techniques".
Source: Helmholtz Centre Potsdam
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