X rays and comets.

Last spring, when astronomers detected an X-ray glow surrounding Comet Hyakutake, theorists scurried to explain the unexpected finding (SN: 6/1/96, p. 346). The ROSAT and Extreme Ultraviolet Explorer satellites have since revealed that Hyakutake is not alone--at least 10 comets, including Hale-Bopp, are now known to exhibit an X-ray glow as they near the sun. Scientists are struggling to account for the behavior.

One theory, proposed by Thomas E. Cravens of the University of Kansas in Lawrence, invokes the solar wind, the stream of charged particles blowing from the sun. He suggests that something special happens when the most highly ionized of those particles, such as oxygen stripped of six of its eight electrons, meets neutral atoms that have boiled off a comet nucleus. The ion steals an electron from the neutral atom and as the captured electron spirals toward the ion, it emits X rays.

Neutral atoms wander far from a comet's core. Cravens contends that his model can thus account for the X-ray glow detected up to 100,000 kilometers from a comet's nucleus. Some researchers argue, however, that this process, known as charge transfer, would produce a steady stream of X rays rather than tire variable emissions observed from Hyakutake.

In a scenario proposed independently by Theodoro G. Northrop of NASA's Goddard Space Flight Center in Greenbelt, Md., and by D.A. Mendis of the University of California, San Diego and his collaborators, high-speed electrons in the interplanetary magnetic field collide with atoms from the comet, causing the electrons to decelerate and emit X rays. Spectra of the X-ray emissions may distinguish between the models.