A research team led by scientists at the Department of Energy's SLAC National Accelerator Laboratory sifted through four years of data from NASA's Fermi Gamma-ray Space Telescope to find the first unambiguous evidence of how cosmic rays -- high-energy particles that bombard the Earth from all directions -- are born.
The researchers identified two ancient supernovas whose shock waves accelerated protons to nearly the speed of light, turning them into what scientists term cosmic rays, a Department of Energy release reported Thursday.
Scientists have theorized the most likely sources for the protons are either supernova explosions within our Milky Way galaxy or powerful jets of energy from black holes outside the galaxy, but lacked the "smoking gun" evidence for either.
"In the last century we've learned a lot about cosmic rays as they arrive here," astrophysicist Stefan Funk said. "We've even had strong suspicions about the source of their acceleration, but we haven't had unambiguous evidence to back them up until recently."
Now that the have the evidence, they said, many questions remain to explain both the way cosmic rays affect life here on Earth, and the fundamental processes that control their origins and acceleration.
"Astronauts have documented that they actually see flashes of light associated with cosmic rays," Funk said. "It's one of the reasons I admire their bravery -- the environment out there is really quite tough."