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Cosmic Ray Source

February 20, 2013

We live in a very energetic universe, as the recent Chelyabinsk meteor demonstrates. This interloper from outer space entered Earth's atmosphere at an estimated 15 - 18 km/sec. Such speed pales in comparison with that of the most energetic bits of matter in the universe, cosmic rays, which travel at nearly the speed of light (300,000 km/sec).

Meteor impact event by Don Davis

As Homer Simpson so famously asked his wife, Marge, "Which b'ak'tun is it?"

This is an artist's conception of the impact of a 1000 kilometer-diameter planetoid on primordial Earth.

(Illustration by Don Davis, via Wikimedia Commons.)

As I've written in a previous article (Local Dark Matter/A Hundred Years of Cosmic Rays, August 17, 2012), cosmic rays were discovered a hundred years ago by the Austrian scientist, Victor Hess. Hess shared the 1936 Nobel Prize in Physics for his discovery.[1] What's surprising is that after a hundred years there are still some unknown things about cosmic rays.

Cosmic rays are nuclei of the elements, usually just hydrogen nuclei (protons), that apparently travel for millions of years at near-light speed before they reach the Earth. Although supernovas have been the leading contenders as cosmic ray emitters, there are quite a few cosmic ray observatories in operation, today, trying to decide, exactly, what's responsible for this isotropic influx of radiation on Earth.[1]

Most scientists believe that galactic cosmic rays originate from supernovas or some process involving black holes, simply because these are the highest energy objects in the galaxy.[3-4,6-7] After all, these cosmic ray particles have energies thousands of time higher than those produced by the Large Hadron Collider.[3-4,6] Detection would be easy, were it not for the complicating influence of the magnetic fields that permeate interstellar space. Protons comprise about 90% of cosmic rays. Since they are charged, their paths are distorted by magnetic fields.[3-4,8]

A new study, four years in the making by 169 scientists from 75 institutions, has tackled the problem by instead looking at a gamma ray signature of cosmic rays.[2-9] Gamma rays are not affected by magnetic fields. The enabling technology for this study was NASA's Fermi Gamma-ray Space Telescope, which was focused on two supernova remnants thousands of light years from Earth.[6-8]

The detection involves the interaction of cosmic ray protons with interstellar material to produce neutral pions. The pions, which are composed of a quark-antiquark pair, quickly decay into two gamma rays at a characteristic energy.[2-5,8] The characteristic energy helps to sort this particular process from others, such as Bremsstrahlung and inverse Compton scattering.[2,4-5]

Supernova Remnant W44

Supernova remnant W44 is seen in this image as the purple patch about 100 light-years across.

(NASA image, Herschel SPIRE/PACS/ESA consortia and ESA/XMM-Newton .)

The necessary data collection was made possible by the Fermi Gamma-ray Space Telescope, launched in June 2008. Fermi is a collaboration between NASA, the US Department of Energy, and scientists in France, Germany, Italy, Japan, and Sweden.[8] Fermi observed two supernova remnants (SNRs), IC 443 and W 44.[2] Supernova remnants IC 443 and W 44 are 5,000 and 10,000 light years distant from Earth, and each of these have strong gamma ray emission in the range expected for pion decay.[3-4,8]

As the study reveals, the acceleration of the cosmic ray particles to near-light speed is not from the supernova explosion, itself, but in the shockwave of the remnant.[3,6] The shockwave compresses the surrounding gas, strengthening the magnetic fields present. Charged particles will traverse the shockwave region repeatedly, gaining energy at each passage, over the course of thousands of years by a process called Fermi acceleration.[3-4,7-8] It's estimated that up to thirty percent of the kinetic energy of the supernova is transferred to cosmic ray protons.[8]

Shock wave of supernova SN 1006

Images of the shockwave region of supernova remnant SN 1006 from the Visible Multi-Object Spectrograph of the European Southern Observatory Very Large Telescope. (European Southern Observatory Image.)[9)]

As the 169 scientists of this study will attest, these data are from the pion decay, not another mechanism, and they validate the SNR shockwave mechanism for cosmic ray acceleration.[5] Are all cosmic ray mysteries now explained? Probably not, since 2011 cosmic-ray observations by the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) instrument aboard the Resurs-DK1 satellite revealed an unexpected rise in cosmic ray flux at very high energies. Cosmic rays might also be accelerated by objects other than SNRs.[4]

In particular, very high energy cosmic rays may emanate from jets associated with black holes.[3] Also, this study was just on galactic cosmic rays, but ultra-high energy cosmic rays are thought to have extragalactic origin.[5]. One example of an extremely energetic cosmic ray is the Oh-My-God particle, detected to have an energy of 3 x 1020 eV. This is 40 million times that of a Large Hadron Collider proton.


  1. Alan Watson, "100 years of cosmic rays," Physics World, Aug 1, 2012.
  2. M. Ackermann, M. et al., "Detection of the Characteristic Pion-Decay Signature in Supernova Remnants," Science, vol. 339, no. 6121 (February 15, 2012), pp. 807-811.
  3. Jonathan Amos, "Cosmic rays: Fermi telescope settles mystery of origin," BBC News, February 14, 2013.
  4. Maggie McKee, "Cosmic rays originate from supernova shockwaves," Nature News, February 15, 2013.
  5. Matthew Francis, "Supernova observations solve the mystery of cosmic-ray origins," Arstechnica, February 14, 2013.
  6. Hayley Dixon, "Scientists solve mystery of cosmic rays," Telegraph (UK), February 15, 2013.
  7. Olivia Moore, "SLAC scientists confirm source of cosmic rays," Stanford Daily, February 15, 2013.
  8. Pete Spotts, "Cosmic rays' mysterious origin? Supernovae to blame, study confirms," Christian Science Monitor, February 14, 2013.
  9. Clues to the Mysterious Origin of Cosmic Rays, European Southern Observatory Press Release No. eso1308, February 14, 2013.

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