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<em>Science</em>: Evidence Shows That Cosmic Rays Come From Exploding Stars

Data collected by the Fermi Space Telescope provide conclusive evidence that supernovae are the source of the speedy, energetic particles called cosmic rays, an international research team reports.

These charged particles, which are mostly protons, continuously assail the planet from outer space. There is general consensus among scientists that supernova remnants (the leftovers of a supernova explosion) are the sources of cosmic rays, but the final proof has been elusive because cosmic rays are deflected on their way to Earth.

A new study offers conclusive evidence that cosmic ray protons within the Milky Way Galaxy are accelerated in the shock waves produced by supernovae.The research appears in the 15 February issue of the journal Science.

Determining the origin of these cosmic rays has “really been one of the big unsolved puzzles in particle astrophysics,” said Stefan Funk, assistant professor of physics at Stanford University, who presented the new results at a 14 Thursday press briefing at the AAAS Annual Meeting in Boston.



In order to understand the origin and acceleration of cosmic ray protons, researchers used data from the Fermi Gamma-ray Space Telescope and targeted W44 and IC 443, two supernova remnants thousands of light years away. | Image courtesy of NASA/DOE/Fermi LAT Collaboration

To confirm this origin, Funk and a team of researchers spent four years (from 2008 to 2012) observing gamma rays with the Large Area Telescope (LAT), which sits onboard the Fermi Gamma-ray Space Telescope. They observed two supernova remnants named IC 433 and W44. Both are located within in the Milky Way Galaxy—IC 443 is roughly 5000 light years away from Earth in the constellation Gemini, while W44 is located about 10,000 light years away, in the constellation Aquila.


The researchers’ targets were pions, subatomic particles produced when accelerated cosmic rays interact with the interstellar material surrounding supernovae. Pions quickly decay into gamma rays which can then be detected with special telescopes.

The problem is that there are multiple processes in the universe that produce gamma rays. When gamma rays enter in a detector, scientists are unable to determine if these rays have been created by high-energy protons or by high-energy electrons.

Analyzing the data, the researchers spotted the characteristic signature of neutral pion decay in the gamma ray spectrum, which unambiguously connects gamma rays to accelerated protons in supernova remnants.

“For the first time were able to detect the ‘smoking gun’ feature of the accelerated protons, that is, the spectral cutoff in the gamma ray spectrum due to the decay of neutral pions,” said Funk.

“Until now, we had only theoretical calculations and common sense to guide us in the belief that cosmic rays were generated in supernova remnants,” said Jerry Ostriker, a Columbia University astrophysicist who was not part of the Science study. “The direct detection of pion-decay signatures in supernova remnants closes the loop and provides dramatic observational evidence for a significant component of cosmic rays.”

“While we have demonstrated that supernova remnants accelerate cosmic rays,” Funk said, “the next step will be to determine exactly they do it, and also up to what energies they can do so.”

At the press briefing, the researchers expressed awe at the link between the demise of giant stars and the tiny particles of cosmic rays—an awe that they said had followed them from throughout their careers.

Supernovae explosions, said Harvard University astrophysicist Patrick Slane, bestow on individual subatomic particles “as much energy as the fastest fastball a pitcher can pitch.”

“These massive explosions,” Funk said, “give energy to the tiniest things we know.”

Read the abstract, “Detection of the Characteristic Pion-Decay Signature in Supernova Remnants,” by M. Ackermann and colleagues.


Ginger Pinholster

Former Director, Office of Public Programs