For decades, physicists have been trying to determine the source of galactic cosmic rays, particles that pervade the Milky Way and could threaten the health of future space travelers. The radiation seems to originate within the debris of exploding stars called supernovae. Now, new observations bolster this notion by showing that the vast plume of matter expelled by a supernova can generate stronger magnetic fields than previously thought, allowing the particles to rev up to spectacular energies.
Supernovae are extremely violent. Debris from an exploding star blasts outward at speeds of millions of kilometers per hour, catching up to slower-moving material that was ejected earlier. The collision creates a shock wave that heats the slower-moving stuff to very high temperatures. Theorists generally agree that the shock waves generate galactic cosmic rays (which are different from even-higher-energy, more-mysterious extragalactic cosmic rays), but they haven't been able to make this scenario work in detail.
Here's the problem: To accelerate the charged particles to the necessary energies, they need to be trapped by magnetic fields, within which they spiral as they gain energy. And physicists had thought that the magnetic fields in supernova remnants were too weak to hold the particles until they reach energies observed in galactic cosmic rays, which are a million times higher than the energies achieved in Earth-bound particle accelerators.
Now, an international team has found otherwise. The supernova shock wave cranks up magnetic fields to 100 times their normal strength, allowing particles to reach higher energies. To prove it, Yasunobu Uchiyama, an astrophysicist at the Japan Aerospace Exploration Agency in Kanagawa, and colleagues used NASA's orbiting Chandra X-ray Observatory to study the remnant of a supernova known as RX J1713.7-3946.
Scrutinizing data collected from 2000 to 2006, the team found spots within the supernova remnant that occasionally glowed extra-brightly in x-rays for periods of about 1 year. By carefully comparing the profiles of those hot spots with theoretical models, the researchers deduced that the x-rays came from electrons swirling inside extremely strong magnetic fields, as they report this week in Nature. The team's conclusion: The shock wave not only accelerates the particles but also intensifies the magnetic fields that tee up the particles for acceleration in the first place. The same mechanism ought to accelerate the protons and atomic nuclei that zoom out into space as galactic cosmic rays.
The researchers have "discovered something very important concerning how supernovae produce cosmic rays, namely that the magnetic field essential for the cosmic ray acceleration is much larger than previously believed," says physicist Donald Ellison of North Carolina State University in Raleigh.
The origins of galactic cosmic rays may not be entirely settled, however. The big question is whether the supernova remnant's magnetic field is powerful enough to also sling out the heavier protons that make up most galactic cosmic rays, Ellison says. The current results suggest that it is, but they don't directly prove it, Ellison says. But Felix Aharonian of the Dublin Institute for Advanced Studies in Ireland, a co-author of the paper, says that gamma ray observations of RX J1713.7-3946 clinch that case, too.
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