LIGO Observations Probe the Dynamics of the Crab Pulsar (Forwarded)

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June 2, 2008

LIGO Observations Probe the Dynamics of the Crab Pulsar

PASADENA, Calif. -- The search for gravitational waves has revealed new
information about the core of one of the most famous objects in the sky:
the Crab Pulsar in the Crab Nebula. An analysis by the international LIGO
(Laser Interferometer Gravitational-Wave Observatory) Scientific
Collaboration to be submitted to Astrophysical Journal Letters has shown
that no more than 4 percent of the energy loss of the pulsar is caused by
the emission of gravitational waves.

The Crab Nebula, located 6,500 light years away in the constellation
Taurus, was formed in a spectacular supernova explosion in 1054. According
to ancient sources, including Chinese texts that referred to it as a
"guest star," the explosion was visible in daylight for more than three
weeks, and may briefly have been brighter than the full moon. At the heart
of the nebula remains an incredibly rapidly spinning neutron star that
sweeps two narrow radio beams across the Earth each time it turns. The
lighthouse-like radio pulses have given the star the name "pulsar."

"The Crab Pulsar is spinning at a rate of 30 times per second. However,
its rotation rate is decreasing rapidly relative to most pulsars,
indicating that it is radiating energy at a prodigious rate," says Graham
Woan of the University of Glasgow, who co-led the science group that used
LIGO data to analyze the Crab Pulsar, along with Michael Landry of the
LIGO Hanford Observatory. Pulsars are almost perfect spheres made up of
neutrons and contain more mass than the sun in an object only 10 km in
radius. The physical mechanisms for energy loss and the accompanying
braking of the pulsar spin rate have been hypothesized to be asymmetric
particle emission, magnetic dipole radiation, and gravitational-wave
emission.

Gravitational waves are ripples in the fabric of space and time and are an
important consequence of Einstein's general theory of relativity. A
perfectly smooth neutron star will not generate gravitational waves as it
spins, but the situation changes if its shape is distorted. Gravitational
waves would have been detectable even if the star were deformed by only a
few meters, which could arise because its semisolid crust is strained or
because its enormous magnetic field distorts it. "The Crab neutron star is
relatively young and therefore expected to be less symmetrical than most,
which means it could generate more gravitational waves," says Graham Woan.

The scenario that gravitational waves significantly brake the Crab pulsar
has been disproved by the new analysis.

Using published timing data about the pulsar rotation rate from the
Jodrell Bank Observatory, LIGO scientists monitored the neutron star from
November 2005 to August 2006 and looked for a synchronous
gravitational-wave signal using data from the three LIGO interferometers,
which were combined to create a single, highly sensitive detector.

The analysis revealed no signs of gravitational waves. But, say the
scientists, this result is itself important because it provides
information about the pulsar and its structure.

"We can now say something definite about the role gravitational waves play
in the dynamics of the Crab Pulsar based on our observations," says David
Reitze, a professor of physics at the University of Florida and
spokesperson for the LIGO Scientific Collaboration. "This is the first
time the spin-down limit has been broken for any pulsar, and this result
is an important milestone for LIGO."

Michael Landry adds, "These results strongly imply that no more than 4
percent of the pulsar's energy loss is due to gravitational radiation. The
remainder of the loss must be due to other mechanisms, such as a
combination of electromagnetic radiation generated by the rapidly rotating
magnetic field of the pulsar and the emission of high-velocity particles
into the nebula."

"LIGO has evolved over many years to its present capability to produce
scientific results of real significance," says Jay Marx of the California
Institute of Technology, LIGO's executive director. "The limit on the Crab
Pulsar's emission of gravitational waves is but one of a number of
important results obtained from LIGO's recent two-year observing period.
These results only serve to further our anticipation for the spectacular
science that will come from LIGO in the coming years."

"Neutron stars are very hot when they are formed in a supernova, and then
they cool rapidly and form a semisolid crust. Our observation of a
relatively young star like the Crab is important because it shows that
this skin, if it had irregularities when it first 'froze,' has by now
become quite smooth," says Bernard F. Schutz, director of the Albert
Einstein Institute in Germany.

Joseph Taylor, a Nobel Prize-winning radio astronomer and professor of
physics at Princeton University, says, "The physics world has been waiting
eagerly for scientific results from LIGO. It is exciting that we now know
something concrete about how nearly spherical a neutron star must be, and
we have definite limits on the strength of its internal magnetic field."

The LIGO project, which is funded by the National Science Foundation, was
designed and is operated by Caltech and the Massachusetts Institute of
Technology for the purpose of detecting gravitational waves, and for the
development of gravitational-wave observations as an astronomical tool.

Research is carried out by the LIGO Scientific Collaboration, a group of
600 scientists at universities around the United States and in 11 foreign
countries. The LIGO Scientific Collaboration interferometer network
includes the LIGO interferometers (including the 2 km and 4 km detectors
in Hanford, Washington, and a 4 km instrument in Livingston, Louisiana)
and the GEO600 interferometer, located in Hannover, Germany, and designed
and operated by scientists from the Max Planck Institute for Gravitational
Physics and partners in the United Kingdom funded by the Science and
Technology Facilities Council (STFC).

The next major milestone for LIGO is the Advanced LIGO Project, slated for
operation in 2014. Advanced LIGO, which will utilize the infrastructure of
the LIGO observatories, will be 10 times more sensitive. Advanced LIGO
will incorporate advanced designs and technologies that have been
developed by the LIGO Scientific Collaboration. It is supported by the
NSF, with additional contributions from the U.K. STFC and the German Max
Planck Gessellschaft.

The increased sensitivity will be important because it will allow
scientists to detect cataclysmic events such as black-hole and
neutron-star collisions at ten-times-greater distances and to search for
much smaller "hills" on the Crab Pulsar.

Related Links

* LIGO
http://www.ligo.caltech.edu/
* GEO
http://www.geo600.uni-hannover.de/
* LIGO's Collaborators
http://www.ligo.org/


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