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  Astronomers get neutron star's measure
The pattern of x-rays generated by neutron stars may reveal their true size, says an international team of astrophysicists.

Research, led by Dr Sudip Bhattacharyya of the Tata Institute of Fundamental Research in India, has revealed an unsuspected property of x-ray bursts given off by the stars.

The research has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

Neutron stars are the densest objects in the universe; a teaspoonful would weigh as much as a mountain.

"Neutron stars provide a natural laboratory to probe matter at a density five to 10 times higher than that of an atomic nucleus. Such studies cannot be done in terrestrial laboratories," says Bhattacharyya.

But neutron stars are also very small - approximately 10 kilometres in diameter - making them difficult to detect.

The only neutron stars we can see are pulsars, which spin rapidly and give off bursts of radiation, similar to a lighthouse.
Size does matter

Co-author of the study Dr Duncan Galloway, of Monash University in Melbourne, says estimating the exact size of neutron stars has proved challenging.

Apart being small and faint, the nearest one is about 1000 light years away.

The researchers used a data set of more than 900 bursts from 43 neutron stars gathered by NASA's Rossi X-ray Timing Explorer satellite.

The bursts occur as the stars consume material from a nearby binary companion, which they orbit.

Neutron stars are so bright that they can radiate as much x-ray energy in one minute as the amount of light radiated by the Sun in approximately one week.

Bhattacharyya and colleagues modelled how the temperature of the bursts changes as they faded and found it varied in relation to the radius of the star.

This change is most likely due to the changing composition of the surface as heavy elements, such as iron, are formed on the star's outer layer.
More revealed

Astronomers have long hoped to use the x-ray bursts to determine the radius of neutron stars more precisely.

Now that this effect is better understood, it is hoped that further analysis will lead to more accurate measurements of the neutron star radius, Galloway says.

For a given mass, the neutron star's radius depends on the star's original composition, he says.

"If we can simultaneously measure the mass and radius for just one neutron star, we can [in principle] determine which of the possible candidate compositions is correct."

Astrophysicist Dr Helen Johnston, of the University of Sydney, says the research is a "nice piece of work."

She says this should allow astronomers to not only measure the size of neutron stars, but to follow the synthesis of heavier elements normally hidden within the star's interior.

"That's seriously cool," she says.
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