Cosmic rays: ASCA finds a superior origin.

Of all the particles that bombard Earth, only cosmic rays provide a direct sample of matter from beyond the solar system. Moreover, these charged particles--electrons and ions that travel at nearly the speed of light--rank as the most energetic known in the universe.

For decades, astronomers have pondered how cosmic rays acquire their enormous energies, which can exceed 1,000 trillion electron-volts, far outstripping the energy produced by any particle accelerator on Earth. Researchers have suspected that the shock wave hurled into space when a massive star explodes as a supernova could provide sufficient acceleration, but they've lacked proof. No one had ever found direct evidence of any but low-energy cosmic rays in the rapidly expanding shell of debris that forms a typical supernova remnant.

Now, high-resolution X-ray observations of the supernova remnant SN 1006, described in the Nov. 16 Nature, indicate that high-energy electrons do originate in the remnant. Two bright regions on opposite sides of the remnant emit X rays in a pattern called synchrotron radiation, which is characteristic of high-speed electrons moving through an interstellar magnetic field. This X-ray signature contrasts with the thermal radiation emitted by hot gases elsewhere in the remnant.

Katsuji Koyama of Kyoto University in Japan, Robert Petre of NASA's Goddard Space Flight Center in Greenbelt, Md., and their colleagues gathered data with the Japanese-U.S. satellite ASCA (Advanced Satellite for Cosmology and Astrophysics). The finding "provides the first strong observational evidence that very high energy cosmic rays are produced in supernova remnant shocks," the team writes.

The shock wave conspires with interstellar magnetic fields to accelerate charged particles. The particles spiral along a magnetic field, but a kink in the field can send them hurtling backward. The particles then bounce like Ping-Pong balls between the field and the shock wave racing up from behind, gaining energy with each bounce.

Confirmation of the ASCA findings may come when the X-ray Timing Explorer, scheduled for launch next month, examines emissions from SN 1006 at higher energies, notes Stephen P. Reynolds of North Carolina State University in Raleigh. Already, the ASCA study "has gone far towards solving at least part of the mystery of cosmic rays," he adds.