Cosmic rays' mysterious origin? Supernovae to blame, study confirms.
A team of researchers found the 'unique, smoking-gun signature' of the creation of cosmic rays in the expanding shells cast off by supernovae.
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Researchers since have refined the idea, with supernova remnants emerging as the most likely breeding ground.Skip to next paragraph
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In essence, the material a supernova sheds expands outward at speeds on the order of 22 million miles per hour, explains Dr. Slane, who was not a member of the team reporting the discovery. As this expanding shell plows through the dust and gas between stars, it sets up a shock wave along its outer edge that heats the gas and carries its own magnetic fields.
The magnetic fields can accelerate already existing cosmic-ray protons. And it can generate news ones by accelerating low-energy protons in the roiling hot soup of ionized gas in the shock wave.
The energy this process imparts to these protons is enormous – perhaps 10 million times higher than the energies achievable at the Large Hadron Collider in Europe, the most powerful Earth-bound particle accelerator.
Indeed, researchers have estimated that anywhere from 3 to 30 percent of the kinetic energy of the star's explosion gets transfered to the protons.
A tiny fraction of these protons escape to become cosmic rays. Others collide with other protons in the shock wave. When collisions occur, they create a short-lived particle known in physics-speak as neutral pions. Each pion immediately decays into two gamma-ray photons. The energy the gamma-ray photons carry represents the smoking gun, since that energy level is determined by the preceding chain of interactions. And, unlike protons, gamma-rays can travel in a straight line. This allowed researcher to pinpoint the sources.
In this case, the sources were the glowing remnant of a supernova about 5,000 light-years away dubbed the Jellyfish Nebula in the constellation Gemini. That star is thought to have exploded sometime between 3,000 and 30,000 years ago. The second source the team studied lies 10,000 light-years away in the constellation Aquila. Called W44, the supernova remnant is expanding within a larger, glowing cloud of dust and gas. The remnant is thought to be about 20,000 years old.
Protons from the aftermath of supernovae throughout the galaxy are though to comprise some 90 percent of the cosmic rays striking Earth's atmosphere. Collisions with the atmosphere generate a shower of other particles that represent the "rays" Hess picked up a century ago.
The Fermi Gamma-ray Space Telescope was launched from the US Air Force's launch complex at Cape Canaveral in Florida in June 2008. It's a cooperative mission spearheaded by NASA and the US Department of Energy, and includes partners in France, Germany, Italy, Japan, and Sweden.
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