WASHINGTON – Superfast protons sometimes slam into the Earth from space flying at close to the speed of light.

Where these particles, called cosmic rays, come from has been a scientific mystery. But new evidence helps confirm the leading explanation – that they originate in the distant remnants of dead stars.

Such a so called supernova remnants contains shells of gas that were ejected from a star before it collapsed in a supernova explosion. They harbor strong magnetic fields that are thought to behave like giant particle accelerators, speeding up particles that become cosmic rays.

New observations from NASA's Fermi Gamma-ray Space Telescope reveal supernova remnants that are emitting radiation a billion times more energetic than visible light. This radiation, which is the short-wavelength gamma ray light, could be a signature of cosmic rays, which are thought to produce gamma rays when they collide with gas.

"Understanding the sources of cosmic rays is one of Fermi's key goals," said Stefan Funk, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the SLAC National Accelerator Laboratory in Menlo Park, Calif. "Fermi now allows us to compare emission from remnants of different ages and in different environments."

Funk presented the findings Monday at a meeting of the American Physical Society in Washington, D.C.
Fermi's Large Area Telescope (LAT) observed gamma ray light from three supernova remnants called W51C, W44, and IC 443, whose stars died between 4,000 and 30,000 years ago. The telescope also spied a much younger remnant, called Cassiopeia A, which is only about 330 years old.

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

Scientists think that the younger supernova remnants have stronger magnetic fields, which are able to hold on to particles long enough to accelerate them to the highest speeds, creating the highest-energy cosmic rays.