We are constantly being bombarded by speedy, energetic, and yet unassuming, particles called cosmic rays. These charged particles (mostly protons), continuously assail the Earth 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 since cosmic rays are deflected on their way from the source to Earth.

A new study offers conclusive evidence that cosmic ray protons within our galaxy are accelerated in the shock waves produced by supernovae. The research appears in the February 15 2013 issue of the journal Science.

"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 Stefan Funk, assistant professor of physics at Stanford University and co-author of the study.

Pions are 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.

Stefan 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 our galaxy - IC 443 is roughly 5,000 light years away from Earth in the constellation Gemini, while W44 is located about 10,000 light years away, in the constellation of Aquila.

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.

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