RELEASE NO: 97-131

SLOWEST KNOWN PULSAR REPORTED

Astronomers announced the discovery of the slowest known isolated pulsar. The object, a small, magnetized and rotating "neutron star," is turning only once every twelve seconds, each turn marked by the receipt of a pulse of X-rays in the vicinity of the Earth. This represents a rotation period 1,000 times slower than expected. Additionally, the majority of such stars discovered to date have been found to emit radio waves, while this object does not.

The X-ray pulses were observed with X-ray telescopes on board the joint Japanese/NASA Advanced Satellite for Cosmology and Astrophysics (ASCA). The finding may reveal an alternate evolutionary path for the remains of originally massive stars (as neutron stars are classified) as well as provide an explanation for "missing" neutron stars, which are expected in the remains of many exploded stars but rarely observed.

These rotating neutron stars are called pulsars, because they are appear to pulse regularly. As the neutron star rotates, beams of light from the poles of the spinning star sweep past the Earth. Astronomers detect this light as regular pulses, rather like people on a ship viewing light from a lighthouse. A normal pulsar pulses up to 600 hundred times per second, because it is rotating very rapidly. The newly discovered pulsar, which appears to be in the center of the supernova remnant known as Kes 73, pulses only once every twelve seconds.

"The fact that this is the longest period observed to date is especially puzzling because Kes 73 is very young, on an astronomical time scale," explained Dr. Gautam Vasisht of the California Institute of Technology in Pasadena, Calif. "Its parent star exploded about 2,000 years ago. Isolated young pulsars tend to slow down as they age, but they do so very slowly. If the pulsar in Kes 73 started out rotating as fast as a normal pulsar, it would require more time than the universe has existed for it to slow down to its present rate."

Scientists have reason to believe that the pulsar in Kes 73 did start out with normal rotation. "Neutron stars are created when a massive star explodes, in a spectacularly violent event called a supernova," said Dr. Eric V. Gotthelf of NASA's Goddard Space Flight Center in Greenbelt, Md. "The core of the star collapses under its own weight while the outer layers are blown off. The core becomes the neutron star, and as it collapses, it spins much more rapidly than the original star because its diameter is a lot smaller."

This is much like what happens when skaters execute a spin and pull their arms in close to their bodies. "If its original rotation was much slower, that means its parent star would have had an extremely slow rotation. This is rather unlikely because of the nature of star formation. This is why we believe the pulsar in Kes 73 started out with a normal rotation," said Gotthelf.

If the Kes 73 pulsar began as a typically rotating neutron star, alternate theories will have to be proposed to explain how it and any others like it evolve. "One explanation is that Kes 73 may be a member of a class of hypothetical neutron stars called magnetars," said Vasisht. "Most neutron stars have intense magnetic fields, more than a trillion times the strength of Earth's field, because the parent star's field becomes extremely concentrated as its core collapses to a neutron star. However, a magnetar's field is stronger still, perhaps up to 100 times stronger than the highest yet known fields. This drag produced by this fantastically strong field would quickly slow the pulsar down to what we see today."

"Another intriguing fact is that we do not see any radio emissions from the pulsar in Kes 73. This may explain why few of the expected neutron stars have been found associated with supernova remnants," added Gotthelf. "Many neutron stars are detected by their characteristic radio emissions. The Kes 73 pulsar may represent a new class of 'radio quiet' neutron stars, and many more may exist, hiding somewhere in the centers of other supernova remains. "

"We hope to understand what make this fascinating object tick so slowly," said Gotthelf. "Future observations with NASA's Advanced X-ray Astrophysics Facility, a space observatory to be launched in 1998, will be very important."

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Editors Note: This press release and images of this observation can be found on the Internet on the Goddard Homepage at the following address:

http://pao.gsfc.nasa.gov/gsfc/newsroom/flash/flash.htm