Cosmic Protons Gone Wild

Supernova remnants act as giant, superefficient particle accelerators.

Credit: ESO/Eveline Helder et al./NASA/Chandra CXC

Shock waves launched into space by a supernova--the explosive death of a giant star--produce cosmic-ray particles carrying tremendous amounts of energy, astronomers have confirmed. The findings, reported today in Science, will give astronomers and physicists a better understanding of some of the universe's more bizarre phenomena.

Astronomers have suspected for more than a decade that supernova shock waves can act like giant particle accelerators. The basic idea is this: As the remnant of a dead star hurtles through space at up to 30 million kilometers per hour, it creates a shock wave as it interacts with the so-called interstellar medium (ISM). Protons in the shock wave get trapped by the magnetic field of the ISM, which bounces the protons back toward the remnant. But the remnant has its own magnetic field, which repels the protons.

Each bounce adds more energy, and eventually the magnetic tennis match accelerates the protons to nearly the speed of light. Knocked free of the remnant and out into deep space, some of the protons finally hit Earth's atmosphere. The particles are so energetic that astronauts have reported seeing flashes of light--caused by single protons striking their retinas--even when their eyes are closed.

Now an international team of astronomers has finally observed the acceleration of protons within a shock wave. Using the Very Large Telescope in Paranal, Chile, and NASA's Chandra spacecraft, they measured the visible light and x-ray emissions of the remnant of a supernova about 8200 light-years away in the direction of the constellation Circinus. These measurements, taken over several years, allowed them to calculate the energies of the protons behind and in front of the shock wave.

The results suggest that the remnant's energy accelerates protons as much as researchers had thought, says physicist and lead author Eveline Helder of Utrecht University in The Netherlands. "We did not expect such a high shock velocity," she says, referring to the speed of the protons in the shock wave. Based on that velocity, the team concludes that more than 50% of the energy of the shock wave must be going to accelerating the protons instead of generating heat.

It's an important paper, says physicist Donald Ellison of North Carolina State University in Raleigh. "It confirms predictions that shocks can be extremely efficient proton accelerators," he says, and it's going to improve understanding "of the physics of the universe's more exotic phenomenon," such as gamma-ray bursts and quasars, which also produce strong shock waves, as well as supernovae.

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