July 10, 2014
On The Hunt For Mysterious Source Of Cosmic Rays
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
All across the Universe high-energy charged particles – mostly protons, electrons, and hydrogen nuclei, though heavier nuclei also exist – are found racing in all directions. The source of these particles, collectively called cosmic rays, is masked by the interstellar magnetic field that bends their paths, making them nearly impossible to directly trace.
Much progress has been made in narrowing down their origin in recent years. The Fermi Gamma-ray Telescope completed an extensive study a couple of years ago, and found that supernova explosions are responsible for accelerating these particles to high energy and thrusting them into the cosmos. However, there is an intrinsic limit to the energies that can be achieved during these events, so the highest energy particles – called ultrahigh-energy cosmic rays – have remained somewhat of a mystery.
Physicists believe the most likely progenitors are Active Galactic Nuclei (AGN). These objects are characterized by supermassive black holes at the cores of massive galaxies that are cannibalizing the system from the inside out. Powerful magnetic fields channel the energy and matter into jets, streaming particles and light across the Universe. Alternatively, some cosmologists believe that Gamma-Ray Bursts (GRBs) – similar to supernovae in emission profile, but on a much larger scale – could be the culprit.
Now, emerging research out of the University of Utah’s massive cosmic ray observatory, has found an interesting artifact near the Big Dipper that may provide clues. "This puts us closer to finding out the sources – but no cigar yet," says University of Utah physicist Gordon Thomson. “All we see is a blob in the sky, and inside this blob there is all sorts of stuff – various types of objects – that could be the source. Now we know where to look."
These cosmic rays are rarer than their lower energy brethren, and also more of a challenge to detect. Once they enter the atmosphere, they interact with atmospheric gas and create air showers – a sequence of gamma-rays and charged particles – that stream towards the ground. Detectors can then detect the cosmic rays and calculate the direction they came from.
Between May 11, 2008 and May 4, 2013, University of Utah’s Telescope Array collected data on these ultra high-energy cosmic rays. During that time, the researchers found only 72 events. But 19 of these were located in the “blob” below the Big Dipper, significantly more than the 4.5 events expected given the size of the region.
"We have a quarter of our events in that circle instead of 6 percent," says Charlie Jui, a University of Utah professor of physics and astronomy. The chances that this result is simply the result of a statistical fluctuation is only 1.4 in 10,000.
Interestingly, a similar experiment in Argentina has found evidence that a similar, though weaker, hotspot exists in the southern hemisphere. They currently don’t have enough data to come to a definitive conclusion, but if real, then the sources of ultra high-energy cosmic rays may have differing sources in the northern and southern hemispheres.
In either case, these initial findings suggest that the sources of these cosmic rays are localized to dense regions of the Universe, where large clusters and super-clusters of galaxies are packed together. Future research will hopefully provide more clues, and eventually put the issue to rest.
This research has been accepted for publication by the Astrophysical Journal Letters.
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