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Fermi closes in
on cosmic rays

Posted: 15 February 2010

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New images from NASA’s Fermi Gamma-ray Space Telescope are helping astronomers take a step closer to uncovering the source of the Universe's most energetic particles – cosmic rays.

Click here for video that shows the supernova remnants and their appearance at different wavelengths of electromagnetic radiation. Credit: NASA/DOE/Fermi LAT Collaboration

Cosmic rays whip through space at nearly the speed of light, deflected by magnetic fields which scrambles their paths and confuses their point of origin. Tell tale signs of their existence are given up when they collide with interstellar gas to produce gamma rays. Understanding the sources of cosmic rays is one of Fermi’s key goals, and the new study focuses on supernova remnants, the best candidates for such high speed processes. The data reveals regions that emit radiation a billion times more energetic than visible light, which has an energy of between two and three electron volts.

“Fermi now allows us to compare emission from remnants of different ages and in different environments,” says Stefan Funkof the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), jointly located at SLAC National Accelerator Laboratory and Stanford University, who presented the findings today at the American Physical Society meeting in Washington, D.C.

Fermi's view of the GeV-gamma-ray emission regions (magenta) in the W44 supernova remnant. The features clearly align with filaments detectable in other wavelengths. This composite merges X-ray data (blue) from the Germany/U.S./UK ROSAT mission, infrared (red) from NASA’s Spitzer Space Telescope, and radio (orange) from the Very Large Array near Socorro, N.M. Image: NASA/DOE/Fermi LAT Collaboration, NASA/ROSAT, NASA/JPL-Caltech, and NRAO/AUI.

The Large Area Telescope (LAT) mapped billion electron volt (GeV) gamma rays from three middle-aged supernova remnants that have never before been resolved at these energies. Each of the remnants, W51C, W44 and IC 443, is the expanding debris shell of a once-massive star that exploded between 4,000 and 30,000 years ago. LAT also confirmed gamma rays emanating from the youthful – just 330 years old – Cassiopeia A (Cas A).

“Older remnants are extremely bright in GeV gamma rays, but relatively faint at higher energies. Younger remnants show a different behavior,” says Yasunobu Uchiyama. “Perhaps the highest-energy cosmic rays have left older remnants, and Fermi sees emission from trapped particles at lower energies.”

A multi-wavelength view of the Cas A supernova remnant. Gamma ray data (in magenta) is from NASA’s Fermi Gamma-ray Space Telescope; X-rays (blue, green) from the NASA’s Chandra X-ray Observatory; visible light (yellow) from the Hubble Space Telescope; infrared (red) from NASA’s Spitzer Space Telescope; and radio (orange) from the Very Large Array near Socorro, N.M. Image: NASA/DOE/Fermi LAT Collaboration, NASA/CXC/SAO, NASA/JPL-Caltech/Steward/O. Krause et al., and NRAO/AUI.

Young supernova remnants seem to possess both stronger magnetic fields and the highest-energy cosmic rays, suggesting that the stronger fields can keep the highest-energy particles in the remnant’s shock wave long enough to speed them to the energies observed. In the new Fermi data, the high energy gamma rays are emanating from locations where the remnants are known to be interacting with cold, dense gas clouds.

“We think that protons accelerated in the remnant are colliding with gas atoms, causing the gamma-ray emission,” says Funk. An alternative explanation is that fast-moving electrons emit gamma rays as they fly past the nuclei of gas atoms. “For now, we can’t distinguish between these possibilities, but we expect that further observations with Fermi will help us to do so.”

Either way, these observations validate the notion that supernova remnants act as enormous accelerators for cosmic particles.

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