Source: NASA/Goddard Space Flight Center
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."
Editor's Note: The original news release can be found here.
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