Mystery of cosmic rays' origin finally solved

The protons that constantly smack into Earth's atmosphere at near the speed of light get their huge energies from exploding stars. At least that's what physicists and astronomers had long suspected, but direct evidence for the idea has been difficult to come by ? until now.

Cosmic rays are any charged particles arriving at Earth from space. Nearly all of them are protons, and some have been accelerated to speeds higher than any achieved by a particle accelerator on Earth. Although we have known about cosmic rays since 1912, their origins have remained a puzzle.

"It's a 100-year-old mystery," says Stefan Funk of the SLAC National Accelerator Laboratory in Menlo Park, California. "What creates these large energies?"

Physicists suspected that one possible source was the violent outburst of a supernova within the Milky Way. The material blown out in the process moves so quickly that it creates a shock wave. Whenever a proton crosses the shock wave boundary, it gets a kick.

Because protons are charged, they can get caught in magnetic fields which carry them back and forth across the shock many times, like a tennis ball bouncing back and forth across a net.

"Eventually their energy gets large enough that they can leave the shock region," Funk says. "Then it is a newborn cosmic ray."

Total scramble

But magnetic fields can also deflect cosmic rays on their way to our detectors. By the time they reach Earth, their directions are totally scrambled, making it hard to determine their origin.

Another approach to the problem was needed ? and gamma rays provided it. We know that when the high-energy protons collide with low-energy protons further out, the violence of the collision indirectly creates gamma rays. These are uncharged and so travel in straight lines, unaffected by magnetic fields.

Funk and colleagues used the Fermi Gamma Ray Space Telescope to observe the two brightest supernova remnants in the galaxy. Would gamma rays from these objects bear the telltale signature of having been made by proton collisions?

Because of conservation of energy, the gamma rays produced during the proton collisions will have a minimum energy of around 150 to 200 megaelectronvolts each, Funk says. If lots of protons are colliding near the supernova remnant, there should be more gamma rays with that energy or higher coming from that region ? and almost none with lower energies

"That's exactly what we see," says Funk. "This is a characteristic, smoking gun feature that absolutely and uniquely tells us that what we are seeing are gamma rays from accelerated protons."

"It is a fairly big deal," says Stefan Westerhoff at the University of Wisconsin-Madison, who was not involved in the work. "That supernova remnants are the sites of cosmic ray acceleration has been suspected for a long time, and there has been indirect evidence for this for a while." The Fermi result "is a very neat measurement and probably pretty much settles the issue", he says.

The finding doesn't explain the origin of all cosmic rays, however. Some are muons or positrons instead of protons ? and one class, the ultra-high energy cosmic rays, are probably from outside our galaxy.

Journal reference: Science, DOI: 10.1126/science.1231160

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