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More cosmic ray excesses reported November 24, 2008

Posted by Sarah in science.
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The Milagro cosmic ray detector

The Milagro cosmic ray detector

In a busy week for cosmic ray science, yet another paper reports the detection of excesses in cosmic ray detections from galactic sources. Using 7 years of data from the Milagro detector, scientists of the University of Maryland and Los Alamos National Laboratory have found two hotspots of high-energy cosmic radiation.

Milagro is a different kind of detector than those I’ve written about before (ATIC and PAMELA). Located in a giant pool of water in New Mexico, it’s designed to detect Cerenkov radiation from the high-energy charged particles that hit the Earth’s atmosphere, causing cascades of high-energy secondary particles as they travel. Cerenkov radiation is the light equivalent of a sonic boom: when a particle travelling close to the speed of light in air enters a medium with a higher refractive index, like water, it can briefly break through the speed of light in that medium as it hits. While the speed of light in a vacuum is a universal constant, it is considerably lower in denser media. The shock of the particle entering the medium causes a cone of photons to be emitted around the particle’s trajectory.

High energy particles like neutrinos or cosmic rays can therefore be detected by the Cerenkov light they emit when hitting water. Large arrays of photomultiplier tubes catch and amplify the faint signal, and from the shape of the cone the direction of the incoming particle can be reconstructed.

So using Milagro, scientists have actually been able to locate the region where the extra particles are originating from.

The excess in particles seen with Milagro also occurs at higher energies than those reported last week: the scientists found the peak of the bump to lie around 10,000 billion electron volts (10 TeV).

The authors basically leave open the question of origin of the observed excesses. While they, like the ATIC team, suggest some nearby galactic source could give rise to the high-energy cosmic rays from the 2 hotspots, there are no known sources in those directions. Another possible source is the heliotail, which is the “downwind” side of the solar wind, where the solar wind plasma connects with the interstellar plasma. Perhaps the inetraction between the magnetic fields at this location can give extra acceleration to cosmic ray particles to cause the large numbers observed with Milagro at one or both of those locations.

Good stuff! The paper, published in the current issue of PRL, is here.


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