Cosmic rays — extremely high-energy particles from space — were first discovered in 1912. Over time, scientists have found that some 90 percent of those particles are protons while the remaining 10 percent are electrons and other atomic nuclei. But they lacked direct evidence of the process that accelerates these particles to such extreme energies.

Now, a study published in the February 15 issue of Science describes the responsible mechanism. A cosmic ray has either a positive or negative charge, and thus its trajectory changes as it traverses the Milky Way’s magnetic fields.

As cosmic rays collide with diffuse gas in the galaxy, they can produce high-energy gamma rays along with other particles. Gamma rays have no electric charge, so magnetic fields don’t affect them.

A cosmic-ray proton colliding with another proton in interstellar gas creates a particle called a neutral pion in addition to other products. That pion then decays into two gamma rays that have a specific energy signature. So, if scientists could find a location where gamma rays of that energy are created, they could determine where cosmic rays originate.

An analysis of four years of data from the Fermi Gamma-ray Space Telescope did just that. Stefan Funk of Stanford University and colleagues studied the gamma-ray signature of two supernova remnants — IC 443 and W44 — and found the characteristic energy signature that results from the decay of neutral pions, and thus from high-energy protons slamming into lower-energy ones.

The Fermi results prove a theory of cosmic-ray acceleration first suggested by physicist Enrico Fermi in 1949. At the front of a supernova’s quicIdy expanding gaseous shell is a shock wave. This shock possesses a strong magnetic field. As a proton crosses that magnetic region, its energy increases by about 1 percent of its original intensity.

A small number of protons will traverse the shock front hundreds of times, boosting their energies and speeds. Occasionally, they will break free from the remnant and travel across the galaxy.

While the two supernova remnants that Funk’s team studied arose from the death of massive stars, type Ia supernovae (white dwarfs that explode once they collect enough material from their companions) also could contribute to cosmic-ray acceleration.

“All you need from the supernova is ejecta moving through the interstellar mate-rial faster than the surrounding material, i.e., all you need is an explosion:’ explains Funk.

According to the researchers, the next step is to determine the details of the acceleration mechanism and the maximum energy that accelerated cosmic-ray protons can attain.

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