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

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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.