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As mentioned in a previous
news article, the neutrino observatory IceCube, situated in the Amundsen-Scott South Pole Station, has been fully operational since last December and its results are being published. One such very interesting result was presented at the recent meeting of the American Physical Society (held in Anaheim, California) by Stefan Westerhoff of the University of Wisconsin-Madison.
IceCube detects muons that are created when neutrinos collide with the ice surrounding its detectors (picture above). Some of these neutrinos come directly from cosmic sources and some from the collision of cosmic rays with the atmosphere.
Between May 2009 and May 2010 (when it was still under construction) IceCube detected 32 billion muons, with a mean energy of 20 TeV, due to cosmic rays.
These cosmic ray muons can be used to derive the direction of arrival of the cosmic ray, and the work of Westerhoff has shown that in the southern sky there are certain directions form which more cosmic rays come. There are therefore some real "hot spots", that produce an excess of cosmic rays, but also regions that produce less.
One of the hot spots appears to coincide with the Vela supernova remnant, a region expected to be a source of cosmic rays about 1000 light years away. The problem, however, is that beyond a distance of about 0.03 light years from the Sun, the Galactic magnetic field should distort the trajectories of the cosmic rays so much that information about their point of origin should be completely lost. Therefore, when we observe the distribution of Galactic cosmic rays we expect to see a very uniform distribution.
Similar hot spots are also seen in the northern hemisphere (where they've been detected by the Milagro Observatory at Los Alamos, New Mexico, and the Tibet Air Shower Array in Yangbajain). At present there is no explanation for the presence of such cosmic ray emission peaks.
Felix Aharonian, of the Dublin Institute for Advanced Studies, Ireland, has proposed the existence of lines of magnetic field that connect these distant sources with our solar system that inhibit the dispersion by the weaker Galactic field. Others have proposed that the cosmic ray sources are actually within our solar system, but in this case the positional coincidence with distant sources is hard to explain. Both ideas seem rather far-fetched, and for the time being we must accept that these cosmic ray sources are simply not understood.
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