4 June 2008

What’s slowing the Crab Pulsar?

Cosmos Online
Like a celestial spinning top, the neutron star known as the Crab Pulsar is slowing. Mysterious gravitational waves had been been fingered as the cause, but a new study reasons that they can't be to blame.
The crab nebula

Speedy, yet slower: The Crab Pulsar, a city-sized, magnetised neutron star spinning 30 times a second, lies at the center of this composite image of the Crab Nebula. The spectacular picture combines optical data (red) from the Hubble Space Telescope and X-ray images (blue) from the Chandra Observatory. Credit: J. Hester (ASU) et al., CXC, HST, NASA

SYDNEY: Like a celestial spinning top, the neutron star known as the Crab Pulsar is slowing. Mysterious gravitational waves had been been fingered as the cause, but a new study reasons that they can’t be to blame.

“We can now say definitively that gravitational waves play only a minor role at best in this phenomenon,” said David Reitze a physicist at the University of Florida in Gainesville, USA. “‘Our measurements tell us that no more than four per cent of the energy loss of the pulsar is caused by the emission of gravitational waves.’”

Supernova brighter then the Moon

Reitze heads up an international team of researchers collaborating on the Laser Interferometer Gravitational Wave Observatory (LIGO) network who detail the evidence refuting gravitational waves in an upcoming Astrophysical Journal Letters.

The Crab Nebula, located 6,500 light years away in the constellation Taurus, was formed in a spectacular supernova explosion that was visible from Earth 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 a rapidly spinning neutron star, or pulsar, that sweeps two narrow radio beams across the Earth each time it turns. Pulsars are tiny, extremely dense and almost perfectly spherical balls of neutrons. The Crab Pulsar itself contains more mass than the Sun, yet has a radius of only 10 km.

“[It] 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,” said Graham Woan of the University of Glasgow in Scotland, who co-led the LIGO science group.

Spin braking

Experts have proposed a number of hypotheses for the physical mechanism behind the spin ‘braking’, including the emission of ‘gravitational waves’. The hypothesis was that the spinning star might generate the waves as a result of even tiny deformations of its shape. Such a deformation might result from physical strain on the pulsar’s semi-solid crust, or from its enormous magnetic field.

These gravitational waves are ripples in the fabric of space and time and are thought to be an important consequence of Einstein’s general theory of relativity.

A perfectly smooth neutron star will not generate gravitational waves as it spins, said the researchers, but the situation changes if its shape is distorted. They reasoned that gravitational waves would have been detectable even if the star were deformed by just a few metres.

“The Crab neutron star is relatively young and therefore expected to be less symmetrical than most, which means it could generate more gravitational waves,” said Woan.

“Eagerly awaited results”

However, the scenario that gravitational waves significantly brake the Crab pulsar has been disproved by his team’s new analysis.

Using published data about the pulsar’s rotation rate from the U.K.’s Jodrell Bank Observatory, LIGO scientists monitored the star from November 2005 to August 2006. The analysis revealed no signs of gravitational waves. But, said the scientists, this result is itself important because it provides information about the pulsar and its structure.

“The physics world has been waiting eagerly for scientific results from LIGO,” commented Joseph Taylor, an astrophysicist at Princeton University in New Jersey, U.S., and winner of a Nobel Prize for the indirect detection of gravitational waves.

“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,” he said.

The LIGO project, which is funded by the U.S. National Science Foundation, was designed and is operated by the California Institute of Technology and the Massachusetts Institute of Technology for the purpose of detecting gravitational waves. Research is carried out by the LIGO Scientific Collaboration, a group of 600 scientists at universities in 12 different countries.

with the University of Florida and the California Institute of Technology.
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