Tikalon Blog by Dev Gualtieri

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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.

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