MYSTERIOUS DELAY OBSERVED IN PULSES FROM BURSTING PULSAR

PRESS RELEASE NO.: 97-76

Astronomers are announcing today they have observed a strange delay in pulses from the Bursting Pulsar, a recently discovered, extremely energetic source near the center of our galaxy.

The report is being presented by Dr. Michael Stark of the University of Maryland and the Goddard Space Flight Center in Greenbelt, Md. to the American Astronomical Society at its summer meeting in Winston-Salem, N.C.

"The result may point us in the right direction to understand the physical phenomena within and around neutron stars, which involve processes not experienced on Earth," said Stark.

The Bursting Pulsar is a so - far unique object that exhibits both rapid pulses of X-ray light and frequent, violent X-ray bursts. The energy released by this object in a single 10 second burst is almost equal to the energy released by the Sun in an entire year.

Data collected by NASA's Rossi X-Ray Timing Explorer (RXTE) spacecraft and analyzed by Dr. Stark, Dr. Tod Strohmayer, Dr. Jean Swank, all of Goddard, and Dr. Altan Baykal of the Middle East Technical University in Ankara, Turkey, show the pulsation from the Bursting Pulsar is delayed each time a burst occurs.

Pulsars are neutron stars which appear to pulse in much the same way as the light from a lighthouse. Neutron stars convert some of the matter which falls into their strong gravitational field into X-rays. They also have a strong magnetic field, which confines the falling matter to small areas on the star's magnetic north and south poles. As a result, X-rays are emitted from these areas in narrow beams. As the star rotates, these beams sweep across the sky and are observed by us as regular pulses.

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Occasionally, some matter in a large disk of matter which has been pulled by the Bursting Pulsar's gravitational field from its nearby companion star builds up and falls in all at once. The infall of this large amount of matter is what causes the X-ray bursts.

"It's as if the inflaming matter that caused the burst turned the pulsar backward," said Stark. "The mystery is that the amount of material falling onto the star isn't large enough to turn a neutron star as far as we have seen, and, in any case, inflaming matter should push the pulsar forward rather than backward."

A possible explanation for these observations is that the magnetic field of the neutron star is distorted by the burst so that the X-ray beam leaves the star in a different direction. Such a contortion could appear to delay the pulsation as seen on Earth. "This explanation isn't adequate because it predicts that the pulsar signal should return to normal soon after the burst, yet the delay persists for hundreds of seconds," said Stark.

A second possible explanation, which may provide clues to the interior of neutron stars, is that the backward push came from inside the star. Inflaming material causes the neutron star to spin faster, much like an ice skater pulling in her arms. However, the core of the star, estimated to be made of a peculiar state of matter called a superfluid, may not be strongly attached to the solid outer crust of the star. When the crust is accelerated by the infalling material, it spins faster than the core. The energy of the burst may cause the crust and the core to stick together momentarily. Since the core is much more massive than the crust, the crust gets "jerked" backward.

Analogous phenomena known as "glitches" by astronomers have been observed in radio pulsars which are slowing down as they radiate energy away as radio waves. However, this is the first time such an effect has been seen in an X-ray pulsar that is speeding up.

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For Additional Information: contact Dr. Michael J. Stark (301 286 4903), Email: Michael.J.Stark@gsfc.nasa.gov