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.
A century after an Austrian scientist discovered evidence for galactic cosmic rays, an international team of researchers has traced their source to exploded stars.Skip to next paragraph
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Such stars, known as supernovae, had previously emerged as prime suspects for generating cosmic rays – something of a misnomer, since galactic cosmic rays are protons that have been accelerated to nearly the speed of light.
But tracing these particles back to their sources is nearly impossible. Protons carry an electrical charge, so magnetic fields they encounter in their travels can deflect them. This can turn what might have been a straight-line path into a torturous one with lots of wide detours.
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Using the Fermi Gamma-ray Space Telescope, researchers instead have detected gamma-ray emissions that are a byproduct of collisions between these turbocharged protons and their sluggish counterparts. The acceleration of these highly-energetic protons and the hapless subatomic crash dummies they hit appear inside the thin shell of material an exploding star casts into space.
Until now, the source of galactic cosmic rays has been one of the biggest unsolved puzzles in particle astrophysics, says Stefan Funk, at the Kavli Institute for Particle Astrophysics and Cosmology at the SLAC National Accelerator Laboratory in Menlo Park, Calif.
But data gathered from two supernova remnants over four years carry "a unique, smoking-gun signature that now, for the first time, provides incontrovertible evidence" that these expanding shells, known as supernova remnants, are accelerating protons, he says.
Dr. Funk notes that cosmic rays not only influence the evolution of galaxies, some biologists suspect they influenced the evolution of life on Earth by introducing mutations into the genes of simple forms of life early in Earth's history. Cosmic rays hitting the top of the atmosphere also trigger a shower of other particles that elevate radiation levels encountered by airliners. The rays also would represent a serious hazard to astronauts en route to Mars.
A formal report of the results will appear in Friday's issue of the journal Science. Funk and Pat Slane, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., reviewed the results in a briefing Thursday at the American Association for the Advancement of Science's 2013 annual meeting in Boston.
Cosmic rays were discovered by physicist Victor Hess in a series of balloon trips he conducted between 1911 and 1913. Filled with hydrogen, the balloons took him to altitudes as high as 17,400 feet.
Researchers had noted the presence of ionizing radiation at ground level, but thought it might have a terrestrial origin. Hess took sensitive detectors on his ascents and found that radiation levels increased with altitude, instead of decreased.
The discovery, confirmed 12 years later by US physicist Robert Millikan, earned Hess a Nobel prize in 1936. Thirteen years later, physicist Enrico Fermi would propose that cosmic rays were accelerated via collisions with moving magnetic fields in interstellar space.