Antarctic Particle Detector Buried in Ice Records Cosmic Ray Weirdness
Detectors buried thousands of feet under the Antarctic ice recently confirmed a mysterious cosmic lopsidedness. Though it might seem reasonable for our planet to receive energetic particles, called cosmic rays, on average from all directions equally, more cosmic rays’ seem to approach Earth from certain preferred directions.
The IceCube Neutrino Observatory, which is still under construction, confirmed these odd cosmic ray preferences, previously detected in the northern hemisphere.
Cosmic rays–energetic particles flung from as nearby as the sun and light years away–are the extra “noise” in the observatory’s experiments; to filter out this noise, researchers needed to map where the cosmic rays are coming from. In a paper published earlier this month in The Astrophysical Journal they confirmed that more cosmic rays seem to come from certain directions–an observation known as anisotropy–in the Earth’s southern hemisphere too.
[T]hey used IceCube to study a longstanding puzzle: whether the distribution of cosmic ray arrivals is uneven across the southern sky, as scientists have previously observed in the northern hemisphere. Indeed, the team found, IceCube detected a disproportionate number of cosmic rays arriving from some parts of the sky. But the reason for this uneven distribution remains unclear. [ScienceNOW]
Physicists designed the detector to search for neutrinos–particles that race through most matter without a trace. The detector picks them up only in the off-chance that they slam into matter, making a brief-lived and only slightly more detectable particle called a muon. The problem is that the detector also picks up muons created when cosmic rays collide with matter in our atmosphere and the muons make their ways into the underground detectors. So the researchers must weed out these cosmic ray-created muons (by studying their paths through the detector). Still, observing the direction of these muons’ arrivals has proven an interesting study on its own.
“IceCube was not built to look at cosmic rays. Cosmic rays are considered background,” said University of Wisconsin-Madison researcher Rasha Abbasi in a statement. “However, we have billions of events of background downward cosmic rays that ended up being very exciting.” [LiveScience]
The completed detector will have almost four times the sensors of the 2007-2008 partially-built system that collected the data for this published study. This may help researchers uncover the cause of this uneven barrage of energetic particles.
The IceCube group is currently extending its analysis to improve its understanding of the anisotropy on a more detailed scale and delve further into its possible causes. While the newly published study used data collected in 2007 and 2008 from just 22 strings of optical detectors in the IceCube telescope, they are now analyzing data from 59 of the 79 strings that are in place to date. When completed in 2011, the National Science Foundation-supported telescope will fill a cubic kilometer of Antarctic ice with 86 strings containing more than 5,000 digital optical sensors. [University of Wisconsin-Madison]
They suspect that the particles may originate from a supernova, such as the nearby supernova remnant Vela, or that perhaps the preferred direction results from interstellar magnetic fields which could steer the particles into these preferred approaches.
“At the beginning, we didn’t know what to expect. To see this anisotropy extending to the Southern Hemisphere sky is an additional piece of the puzzle around this enigmatic effect — whether it’s due to the magnetic field surrounding us or to the effect of a nearby supernova remnant, we don’t know,” Abbasi says…. “This is exciting because this effect could be the ’smoking gun’ for our long-sought understanding of the source of high-energy cosmic rays.” [University of Wisconsin-Madison]
Image: University of Wisconsin-Madison / IceCube Neutrino Observatory
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