IceCube spies unexplained pattern of cosmic rays

Jul 27, 2010 by Jill Sakai
This "skymap," generated in 2009 from data collected by the IceCube Neutrino Observatory, shows the relative intensity of cosmic rays directed toward the Earth’s Southern Hemisphere. Researchers from UW-Madison and elsewhere identified an unusual pattern of cosmic rays, with an excess (warmer colors) detected in one part of the sky and a deficit (cooler colors) in another. Photo: courtesy IceCube collaboration

(PhysOrg.com) -- Though still under construction, the IceCube Neutrino Observatory at the South Pole is already delivering scientific results - including an early finding about a phenomenon the telescope was not even designed to study.

IceCube captures signals of notoriously elusive but scientifically fascinating called neutrinos. The focuses on high-energy neutrinos that travel through the Earth, providing information about faraway cosmic events such as supernovas and in the part of space visible from the Northern Hemisphere.

However, one of the challenges of detecting these relatively rare particles is that the telescope is constantly bombarded by other particles, including many generated by interacting with the Earth's atmosphere over the southern half of the sky. For most IceCube neutrino physicists these particles are simply background noise, but University of Wisconsin-Madison researchers Rasha Abbasi and Paolo Desiati, with collaborator Juan Carlos Díaz-Vélez, recognized an opportunity in the cosmic ray data.

"IceCube was not built to look at cosmic rays. Cosmic rays are considered background," Abbasi says. "However, we have billions of events of background downward cosmic rays that ended up being very exciting."

Abbasi saw an unusual pattern when she looked at a "skymap" of the relative intensity of cosmic rays directed toward the Earth's Southern Hemisphere, with an excess of cosmic rays detected in one part of the sky and a deficit in another. A similar lopsidedness, called "anisotropy," has been seen from the Northern Hemisphere by previous experiments, she says, but its source is still a mystery.

"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.

The new result publishes Aug. 1 in The Astrophysical Journal Letters, published by the American Astronomical Society.

One possible explanation for the irregular pattern is the remains of an exploded supernova, such as the relatively young nearby supernova remnant Vela, whose location corresponds to one of the cosmic ray hotspots in the anisotropy skymap. The pattern of cosmic rays also reveals more detail about the interstellar magnetic fields produced by moving gases of charged particles near Earth, which are difficult to study and poorly understood.

Right now "we can predict some models, but we don't have concrete knowledge of the magnetic field on small scales," Abbasi says. "It would be really nice if we did — we would have made a lot more progress in the field."

Since nearly all cosmic signals are influenced by the interstellar magnetic fields, a better overall picture of these fields would aid a large range of physics and astronomy studies, she says, adding that their newly reported findings rule out some proposed theories about the source of the anisotropy.

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.

"This is exciting because this effect could be the 'smoking gun' for our long-sought understanding of the source of high-energy cosmic rays," says Abbasi.

Explore further: Gravitational waves according to Planck

More information: icecube.wisc.edu/

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omatumr
3 / 5 (3) Jul 28, 2010
The asymmetric distribution of cosmic rays is indeed intriguing, if confirmed.

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo
SteveL
5 / 5 (1) Jul 28, 2010
I wonder what kind of time is involved here. This is data from 2007 and 2008. Are these quick scans over a short period of time, scans that take a long time to perform, or a compilation of many scans during these time periods? I don't see this in the report. You kind of have to know how the data was gathered to have any ability to understand what kind of sources to look at. The earth is in a dynamic system, so the variables are continuiously changing. Not knowing how the results of samples correlate to the dynamic variables at a specific time in a process makes for a bit of a challenge. What were the sensors astronomically pointed at when they detected the muons?

I followed the link - no additional info there.
hbar_squared
4.7 / 5 (3) Jul 28, 2010
@SteveL

Just from what I know about IceCube, I would guess it is a compliation of data collected over the two year span. The observatory is very similar to a cloud chamber - it is not 'aimed', but particles are tracked as they pass through the volume, and their trajectory is reconstructed from the correlated events. It is unlikely that they collect enough data in a small amount of time to make this analysis.

IceCube is a static observatory, so it is not 'aimed' at anything. However, it does have a 360 degree field of view*. That makes constructing skymaps like the one above very easy.

*The Earth blocks half of that, but in this observatory that is a benefit, because neutrinos can pass through the Earth while everything else is blocked.
omatumr
1 / 5 (2) Jul 30, 2010
Why the uneven distribution pattern?

My first guess is that the distribution of massive, compact stellar objects - energized by neutron repulsion ["Attraction and repulsion of nucleons: Sources of stellar energy", Journal of Fusion Energy 19, 93-98 (2001)] - has a similar distribution pattern.

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo
StandingBear
3 / 5 (2) Aug 03, 2010
The noted anisotropic distribution of cosmic rays may over time point not to a '360' degree view which alludes to a planar or two dimensional view, but actually a spherical view with the spherical coordinates' limits: (rho,theta)= (0-360,0-360). In this matter, all anisotropic intensity signatures may not only have entrance but also exit signatures. The aggregate of this may prove to look messy, but fourier sums of aggregate data will point to the sum of historic stellar death remnants over eons of time. This may ultimately prove to contribute significant local affects to percieved universal phenomena.