Evidence emerges that intense stellar explosions send high-energy protons hurtling through the galaxy

By Andrew Grant

Web edition: February 14, 2013
Print edition: March 23, 2013; Vol.183 #6 (p. 16)

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The supernova remnant known as W44 (shown in purple, combining infrared and X-ray observations) helped researchers find evidence that supernovas produce cosmic rays.

Credit: ESA/Herschel/HOBYS/XMM-Newton

Remnants of two supernovas have provided the best proof yet that supernova shock waves churn out the high-energy protons that whizz through the galaxy.

Many of these speedy protons, better known as cosmic rays, have more energy than those in any manmade particle accelerator. Astronomers spent four years probing W44, the remains of a supernova located about 10,000 light-years away in the constellation Aquila, and another remnant called IC 433, located roughly 5,000 light-years away in the constellation Gemini, with NASA’s Fermi Gamma-ray Space Telescope. The scope picked up a steady stream of gamma radiation at energies that could only have come from a nuclear reaction involving cosmic rays slamming into slower-moving interstellar material, researchers report in the Feb. 15 Science.

Scientists have long suspected that protons become cosmic rays when they get energy boosts from interacting with rapidly expanding supernova shock waves. But in the absence of strong observational evidence, some skeptics had proposed other mechanisms. “This measurement puts those theories to rest,” says Stanford University astrophysicist and study author Stefan Funk. “This is definitive proof that protons are accelerated in supernova remnants.”