Neutron Stars on Ice

SEATTLE--Astronomers studying bursts of x-rays from spinning neutron stars may have seen one of these superdense objects brake and accelerate like a spinning figure skater, all in the space of a few seconds. That's frisky behavior for an object that packs the mass of the sun into a diameter of a few tens of kilometers. Then again, the behavior--described at the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW)--is powered by a vast thermonuclear explosion, triggered when a companion star dumps material onto the neutron star's surface.

This scenario could solve a puzzle that arose a year ago, soon after NASA's X-ray Timing Explorer (XTE) satellite began clocking the flickers of x-rays coming from so-called x-ray binaries. X-ray binaries often consist of a neutron star and an ordinary companion star, which sheds material onto the neutron star. The infalling material triggers bursts of fusion near the surface of the neutron star, fueling its bursts of x-rays. Theorists also believed that the infalling material, spiraling onto the neutron star, sets it spinning rapidly. They predicted that XTE measurements would reveal this rapid rotation as the x-ray "hot spots" whirled into view and away again.

Last year, Tod Strohmayer, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues picked up what seemed to be a signal of rotation when they pointed the XTE toward a neutron star called 4U 1728-34. They found that the number of x-rays collected during bursts oscillate at a frequency of 363 cycles per second. The problem was that the oscillations varied by a few cycles per second. Something as massive as a neutron star should not change its rotational speed that much in a few seconds, says theorist Lars Bildsten of the University of California, Berkeley.

Now, with more detailed x-ray observations, Strohmayer has a simple explanation. The new data show that the number of cycles per second drops in the first second, then recovers over the next few seconds. The reason, he believes, is that when the infalling matter triggers a thermonuclear detonation, the reaction transfers heat to the atmosphere of the star, which inflates slightly. Like a figure skater extending her arms, the atmosphere rotates a little more slowly. Then, as the blasted plasma cools, the atmospheric layer contracts and returns to its higher spin rate. Throughout this swelling, Strohmayer speculates, the star continues to rotate at 363 cycles per second.

Other astrophysicists like the idea. Says Michiel van der Klis of the University of Amsterdam in the Netherlands, "It has a nice, simple ring to it."

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