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LIGO Observations Probe Dynamics of the Crab Pulsar

The search for gravitational waves has revealed new information about the core of one of the most famous objects in the sky, providing a glimpse of the spectacular discoveries that may come from the Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration.

The LIGO collaboration, which uses huge detectors in Washington and Louisiana to search for gravitational waves, studied the Crab Pulsar in the Crab Nebula, a popular target for amateur astronomers. The analysis has shown that no more than 4 percent of the pulsar’s energy loss is caused by the emission of gravitational waves, disproving one theory of what is slowing the pulsar’s spin rate.

“The Crab Pulsar is spinning at a rate of 30 times per second,” said Graham Woan of the University of Glasgow. “However, its rotation rate is decreasing rapidly relative to most pulsars, indicating that it is radiating energy at a prodigious rate.”

Woan co-led the science group that used LIGO data to analyze the Crab Pulsar, along with Michael Landry of the LIGO Hanford (Wash.) Observatory. Their findings have been submitted to the journal Astrophysical Journal Letters.

The LIGO Scientific Collaboration is a group of 600 scientists at universities around the United States and in 11 foreign countries. The University of Mississippi is a member of the collaboration’s Compact Binary Coalescence Group, which studies the detection of gravitational waves from the inspiral and merger of binary compact stars and black holes. The Crab Pulsar analysis was performed by the collaboration’s Continuous Wave Group, which studies the detection of gravitational waves from rotating compact objects, such as pulsars and neutron stars.

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 Chinese texts, 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.”


Pulsars are almost perfect spheres made up of neutrons and contain more mass than the sun in an object only 10 kilometers 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,” Woan said.

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 this result is itself important because it provides information about the pulsar and its structure, said David Reitze, professor of physics at the University of Florida and spokesperson for the LIGO Scientific Collaboration.

“We can now say something definite about the role gravitational waves play in the dynamics of the Crab Pulsar based on our observations,” Reitze said. “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 added, “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.”

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.

The collaboration’s interferometer network includes the LIGO interferometers (including the 2-kilometer and 4-kilometer detectors in Hanford, Wash., and a 4-kilometer instrument in Livingston, La.) 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.

Work on LIGO began in the 1970s, and ground was broken on the first observatory in 1994.

“After so many years of dedicated work by many scientists, it is now time for scientific payoffs,” said Marco Cavaglia, UM assistant professor of physics and astronomy and principal investigator of the Ole Miss LIGO team. “The latest result by the LIGO Scientific Collaboration is very important because it will allow us to better understand the structure of neutron stars.”

LIGO has evolved to its present capability to produce significant scientific results, said 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,” Marx said. “These results only serve to further our anticipation for the spectacular science that will come from LIGO in the coming years.”

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