Scientists have cracked a long-standing mystery surrounding the source of cosmic rays, a form of high-energy radiation emanating from deep space that continually crashes into the earth's atmosphere.
The rays, mostly made up of high-energy protons, stream through the galaxy and interact with air molecules high in the earth's upper atmosphere, creating showers of charged particles, some of which reach the ground.
Though ubiquitous, the origin of cosmic rays has never been pinned down. Scientists have suggested that they could either come from the remains of supernovae, which are the violent explosions that mark the death of the biggest stars, or they could be particles accelerated by black holes somewhere outside our galaxy.
"The energies of these protons are far beyond what the most powerful particle colliders on earth can produce," said Professor Stefan Funk, an astrophysicist with the Kavli Institute and Stanford University, who led the latest work. "In the last century, we've learned a lot about cosmic rays as they arrive here. 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."
People on the ground are shielded from any harmful effects of cosmic rays thanks to the earth's thick atmosphere. The higher up you go - flying in a plane, for example - the higher the dose of radiation you receive.
The greatest hazards from cosmic rays, however, are faced by astronauts floating above the earth - and possibly one day travelling to Mars. The doses of radiation individuals would experience on a long-duration space flight are one of the major limiting factors to human missions beyond earth.
Funk's team examined four years of data from the Fermi gamma-ray space telescope, which had recorded the radiation coming from two supernova remnants in our galaxy: IC 443, which is around 5,000 light years away from earth in the constellation Gemini, and W44, which is around 10,000 light years away in the constellation of Aquila.
In the data, they found evidence for gamma rays that had come from the decay of a type of particle, called a neutral pion, which is created when cosmic rays interact with the material surrounding a supernova.
The results were published in the journal Science and were announced at the annual meeting of the American Association for the Advancement of Science in Boston.
The supernovae accelerate protons to almost the speed of light and shoot them in all directions. Some end up at earth and we call them cosmic rays.
Though Funk's team has found a source for the cosmic rays, he said the work on their origins was far from complete. "We would like to understand the acceleration mechanism in more detail, to be able to better understand some of the most energetic processes in our galaxy," he said.
Another question is whether the majority of cosmic rays come from supernovae, or just some of them. "We have taken the first crucial step of demonstrating that cosmic rays are accelerated in supernova remnants, but there is still work to be done to fully understand the origin of the cosmic rays," Funk said.