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Milky Way Object Sends X-rays in Pulses Plus Bursts

February 28, 1996

NEW YORK (AP) _ Astronomers have found an object in the Milky Way galaxy that sends X-rays toward Earth both in regular pulses and huge bursts, the first time such a combination has been observed.

The object is thought to be a super-dense neutron star paired with a less massive star. Further study might shed light on how neutron stars evolve in such ``binary″ systems, said researcher Chryssa Kouveliotou.

The object is in the direction of the galaxy’s center, in Earth’s southern sky. Its discovery was reported in Thursday’s issue of the journal Nature by Kouveliotou, of the Universities Space Research Association at the NASA’s Marshall Space Flight Center in Huntsville, Ala., and scientists elsewhere.

Scientists call the object a bursting pulsar. The unusually strong bursts of X-rays, carrying about 1 million times the power of the Sun, appear about 20 times or more a day. The pulses are detected about twice per second.

As with other pulsars, the pulses are thought to be caused by a spinning of the neutron star. The star sends out a continuous beam of energy that sweeps past Earth twice a second, like the rotating beam of a lighthouse.

The powerful bursts of X-rays appear to be caused by a sudden dumping of matter from the companion star onto the surface of the neutron star, Kouveliotou said.

The neutron star sucks the material away from its companion by gravity or magnetic force, and the material then forms a disk that orbits the neutron star, she said. The bursts occur when, for unknown reasons, chunks of material fall from the disk onto the neutron star, she said.

That would produce X-rays because it would heat the material to about 1 billion degrees.

More than 1,000 X-ray bursts from the object have been detected since they were first observed Dec. 2 by NASA’s Compton Gamma Ray Observatory spacecraft.

The object may represent a relatively short-lived stage, and the bursts may peter out soon, so scientists are hustling to study them, said Stephen Maran of NASA’s Goddard Space Flight Center in Greenbelt, Md.

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