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Stellar X-rays burst brings theory shock.

Stellar X-ray burst brings theory shock

A spectacular burst of X-rays from a compact star system has shattered a painstakingly assembled picture--based on a decade of observations -- that astronomers thought accounted for the source's peculiar characteristics. The occurrence of such an outburst, detected by Japan's Ginga X-ray astronomy satellite, raises serious questions about how well astronomers understand what happens when a neutron star collects, or accretes, matter from an orbiting companion star.

The long and bright outburst was the first ever observed emanating from the X-ray source designated AC211, which lies at the center of the globular cluster M15,a region of densely packed stars on the Milky Way's edge. Tadayasu Dotani of the Institute of Space and Astronautical Science in Kanagawa, Japan, and his co-workers describe the surprising satellite data in the Oct. 11 NATURE.

"It's an important discovery," says astronomer Charles D. Bailyn of Yale University in New Haven, Conn. "It opens a whole can of worms . . . which we had hoped we had put the lid."

X-ray bursts apparently occur when sufficient material collects on a neutron star's surface to initiate a runaway thermonuclear reaction. The newly arrived surface material periodically ignites, typically within about a second, and burns for a few hundred seconds, releasing copious quantities of X-rays.

Until the Ginga data showed otherwise, AC211 was the only one of nine known accreting neutron stars in globular clusters that had seemingly failed to exhibit this process. Astronomers theorized that AC211 collected matter so quickly that the material burned steadily as it landed, rather than building up and then exploding. And to explain why astronomers observed a low l evel of total X-ray emissions despite this steady thermonuclear activity, they assumed that the disk of material surrounding the neutron star obscured the view, in effect shielding Earth-based obseervers from the bulk of X-rays released.

"Now that this burst has been seen, essentially all such theories are pretty much out the window," Bailyn says.

If an accretion disk partially blocks the view, the intensity of any detected X-ray burst represents only a fraction of its true intensity. But the intensity of the X-ray burst observed by Ginga was already significantly higher than the theoretical level at which radiation pressure halts further accretion.

"The idea that the accretion disk blocks our view of the central neutron star must be wrong," says Michael R. Garcia of the Center for Astrophysics in Cambridge, Mass. "The only way to salvage it is to assume that the disk changes shape during the burst, or that the burst was from another, previously unknown source within a [short distance] of AC211."

But those possibilities and several other theories raise more questions than they answer. "Doubtless, theorists will soon provide us with theories about what can happen, but we don't have them at the moment," Bailyn says.

The peculiar behavior of AC211 "is yet another indication that this whole class of objects -- accreting compact objects -- is very complicated," he adds. "Although we have a basic understanding of what is going on, the details constantly surprise us."
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Title Annotation:star system AC211
Author:Peterson, Ivars
Publication:Science News
Date:Oct 20, 1990
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