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Nancy Neal
Headquarters, Washington                        July 2, 2003
(Phone: 202/358-1547)

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-5017)

RELEASE: 03-224

EINSTEIN'S GRAVITATIONAL WAVES MAY SET SPEED LIMIT FOR PULSAR 
SPIN 

     Gravitational radiation, ripples in the fabric of space 
predicted by Albert Einstein, may serve as a cosmic traffic 
enforcer, protecting reckless pulsars from spinning too fast 
and blowing apart, according to a report published in the 
July 3 issue of Nature.

Pulsars, the fastest spinning stars in the Universe, are the 
core remains of exploded stars, containing the mass of our 
Sun compressed into a sphere about 10 miles across. Some 
pulsars gain speed by pulling in gas from a neighboring star, 
reaching spin rates of nearly one revolution per millisecond, 
or almost 20 percent light speed. These "millisecond" pulsars 
would fly apart if they gained much more speed.

Using NASA's Rossi X-ray Timing Explorer, scientists have 
found a limit to how fast a pulsar spins and speculate that 
the cause is gravitational radiation: The faster a pulsar 
spins, the more gravitational radiation it might release, as 
its exquisite spherical shape becomes slightly deformed. This 
may restrain the pulsar's rotation and save it from 
obliteration.

"Nature has set a speed limit for pulsar spins," said Prof. 
Deepto Chakrabarty of the Massachusetts Institute of 
Technology (MIT) in Cambridge, lead author on the journal 
article. "Just like cars speeding on a highway, the fastest-
spinning pulsars could technically go twice as fast, but 
something stops them before they break apart. It may be 
gravitational radiation that prevents pulsars from destroying 
themselves."

Chakrabarty's co-authors are Drs. Edward Morgan, Michael 
Muno, and Duncan Galloway of MIT; Rudy Wijnands, University 
of St. Andrews, Scotland; Michiel van der Klis, University of 
Amsterdam; and Craig Markwardt, NASA Goddard Space Flight 
Center, Greenbelt, Md. Wijnands also leads a second Nature 
letter complementing this finding.

Gravitational waves, analogous to waves upon an ocean, are 
ripples in four-dimensional spacetime. These exotic waves, 
predicted by Einstein's theory of relativity, are produced by 
massive objects in motion and have not yet been directly 
detected.

Created in a star explosion, a pulsar is born spinning, 
perhaps 30 times per second, and slows down over millions of 
years. Yet if the dense pulsar, with its strong gravitational 
potential, is in a binary system, it can pull in material 
from its companion star. This influx can spin up the pulsar 
to the millisecond range, rotating hundreds of times per 
second.

In some pulsars, the accumulating material on the surface 
occasionally is consumed in a massive thermonuclear 
explosion, emitting a burst of X-ray light lasting only a few 
seconds. In this fury lies a brief opportunity to measure the 
spin of otherwise faint pulsars. Scientists report in Nature 
that a type of flickering found in these X-ray bursts, called 
"burst oscillations," serves as a direct measure of the 
pulsars' spin rate. Studying the burst oscillations from 11 
pulsars, they found none spinning faster than 619 times per 
second.

The Rossi Explorer is capable of detecting pulsars spinning 
as fast as 4,000 times per second. Pulsar breakup is 
predicted to occur at 1,000 to 3,000 revolutions per second. 
Yet scientists have found none that fast. From statistical 
analysis of 11 pulsars, they concluded that the maximum speed 
seen in nature must be below 760 revolutions per second.

This observation supports the theory of a feedback mechanism 
involving gravitational radiation limiting pulsar speeds, 
proposed by Prof. Lars Bildsten of the University of 
California, Santa Barbara. As the pulsar picks up speed 
through accretion, any slight distortion in the star's dense, 
half-mile-thick crust of crystalline metal will allow the 
pulsar to radiate gravitational waves. (Envision a spinning, 
oblong rugby ball in water, which would cause more ripples 
than a spinning, spherical basketball.) An equilibrium 
rotation rate is eventually reached where the angular 
momentum shed by emitting gravitational radiation matches the 
angular momentum being added to the pulsar by its companion 
star.

Bildsten said that accreting millisecond pulsars could 
eventually be studied in greater detail in an entirely new 
way, through the direct detection of their gravitational 
radiation. LIGO, the Laser Interferometer Gravitational-Wave 
Observatory now in operation in Hanford, Wash. and in 
Livingston, La., will eventually be tunable to the frequency 
at which millisecond pulsars are expected to emit 
gravitational waves.

"The waves are subtle, altering spacetime and the distance 
between objects as far apart as the Earth and the Moon by 
much less than the width of an atom," said Prof. Barry 
Barish, LIGO director from the California Institute of 
Technology, Pasadena. "As such, gravitational radiation has 
not been directly detected yet. We hope to change that soon." 
For animation, images and more information, visit the 
Internet at:

http://www.gsfc.nasa.gov/topstory/2003/0702pulsarspeed.html

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