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Star twins

The first double-team of pulsars may reveal secrets of gravity.
9 January 2004

MARK PEPLOW

Artists impression of the evolution of the binary pulsars.
animation

Astronomers have realized that a rare set of double stars is made up of two pulsars1. This unique discovery will allow them to test Einstein's theory of relativity in novel ways, and to better understand the energy beams that pulsars generate.

"This is a hugely significant discovery," says Robert Massey of the Royal Observatory, Greenwich, in London, UK. Einstein predicted the existence of gravitational waves, but they have never been directly observed. "There aren't many objects out there that could be a copious enough source of gravitational waves, but this is one of them," he says.

An international team of scientists first published information about the binary system in December2, but they thought it contained one pulsar and one neutron star, a relatively exotic combination in itself. Pulsars - spinning stars that emit directed beams of light and radiation - are often called the lighthouses of the cosmos. Neutron stars are the incredibly dense debris left after a star explodes as a supernova. Discovering that the smaller neutron star was also a pulsar makes the system even more unusual.

Their initial mistake was simply a matter of bad luck, says Andrew Lyne, director of Jodrell Bank Observatory in Macclesfield, UK, and lead author on the initial study. As the two pulsars spiral around each other, the larger star blasts away at its sibling's magnetic field, distorting its radiation beam. "We only spent about four minutes looking at each part of the sky. It was just bad luck that the first time we looked at the system, the smaller pulsar was too dim to see," says Lyne.

While the larger pulsar rotates once every 23 milliseconds, its partner makes just one rotation every 2.8 seconds - slightly slower than a record on a turntable.

The pulsars in their current state.
animation

Lyne and his colleagues think that the system was formed by the larger pulsar sucking matter away from a companion star, spinning faster and faster as it did so. The companion then turned into a second, smaller pulsar after a supernova explosion.

Binaries such as these are thought to emit gravitational waves as the stars circle one another, losing energy until they meet in an intense collision. The end of the pulsars' dance of death is expected in about 85 million years time, and will be marked by one last, enormous burst of gravity waves.

Four different effects beyond those explained by simple newtonian gravity have already been measured using the binary stars and are consistent with Einstein's theory of gravity.

The orientation of the two pulsars means that the radiation from one shines through the magnetic field of the other, giving scientists an unprecedented opportunity to work out what happens in the region immediately around a pulsar. "In the past we've only been able to guess at things like this," says Lyne. "Now we can actually observe it."

References
  1. Lyne, A. G. et al. A Double pulsar system - A Rare Laboratory for Relativistic Gravity and Plasma Physics. Science, doi:10.1126/science.1094645 (2004). |Article|
  2. Burgay, M. et al. An increased estimate of the merger rate of double neutron stars from observations of a highly relativistic system. Nature, 426, 531 - 533, doi:10.1038/nature02124 (2004). |Article|


Nature News Service / Macmillan Magazines Ltd 2004

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