Cosmic Rays May Not Come From Supernovas
The confirmed origin of ordinary cosmic rays may need to be unconfirmed.
New data gathered by an instrument onboard a Russian spacecraft challenge the theory that most cosmic rays are fueled by supernovas, the explosions created by dying stars.
“The mechanism for the acceleration of cosmic rays needs to be completely revised,” says Piergiorgio Picozza, a physicist at the University of Rome Tor Vergata in Italy. Picozza is a co-author of a March 3 paper in Science detailing the new observations of the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, or PAMELA, instrument.
Cosmic rays aren’t actually rays. They’re fast-moving particles that carry an extraordinary amount of energy and continuously bombard the Earth from every direction. The most popular explanation for the origin of these particles points to shock waves created by far-off supernovas, one of the few phenomena in the cosmos powerful enough to impart such energy.
According to that explanation, known as the diffusive shock acceleration mechanism, clouds of charged gas rush outward during a supernova and generate strong magnetic fields. These magnetic fields could accelerate charged particles to tremendous speeds and eject them into space.
Orbiting hundreds of kilometers above Earth, the PAMELA detector spent three years collecting cosmic ray particles; mostly nuclei of hydrogen and helium with energies ranging from a billion to a trillion electron volts, which is comparable to the energy of protons in the biggest particle accelerator in the United States.
Magnetic fields in a supernova should accelerate both hydrogen and helium particles in the same way: Graph the mathematical equations describing this push, and the curve for each particle should have the same slope. But in the PAMELA data, Picozza found a difference in these slopes that a single shock wave can’t explain.
“The two particles seem to be accelerated by different mechanisms,” he says.
Scientists should investigate other astronomical objects as possible sources of cosmic rays, Picozza says. One place to look proposed by Russian physicists is in the novas, or smaller explosions, produced when white dwarf stars belch out energy. Another option is giant superbubbles of gas blown around the universe by stellar winds, says Picozza.
But Mikhail Malkov, a plasma physicist at the University of California, San Diego, who studies supernova shock waves, isn’t ready to toss out the existing cosmic ray theory. “The data look statistically significant, but it’s too early to say that the supernova acceleration model is in trouble. This statement is too strong,” says Malkov.
Space telescopes peering into the remnants of supernovas have found lots of evidence over the years to support the supernova shock wave theory — including gamma rays that reveal the structure of magnetic fields, and missing energy that could have been spent making cosmic rays.
Malkov says the difference between Picozza’s hydrogen and helium curves is small, and it could be accounted for simply by tweaking the existing supernova model. Malkov hasn’t worked out the details yet, but he suspects that PAMELA may be seeing cosmic rays created by a shock wave that wasn’t completely uniform or a mishmash of particles released by two different supernovas.