Nuke blasts reveal true size of neutron stars

Satellite data and new modelling find neutron stars are less than 25 kilometres wide. Andrew Masterson reports.

Thermonuclear blasts on a neutron star give away its size and mass.

New modelling of the thermonuclear blasts happening on the surface of a neutron star means that astrophysicists have now determined the size of the super-dense bodies to within an astonishing 400 metres.

Neutron stars are the core remnants of large stars that explode as supernovae. In October this year, teams of scientists announced the first ever recording of two of them colliding, producing a rarely detected gravitational wave.

It has been known for years that neutron stars are very small, with a radius of between 10 to 20 kilometres. They are also extremely dense. A cubic centimetre of one is estimated to weigh in the region of 100 million tonnes.

The ongoing quest to better understand the properties, shape and behaviour of the gravitational wave recently detected is highly influenced by knowing the size and density of the neutron stars that set it in motion when they smacked into each other 130 million light years away.

For this reason, work led by Joonas Nättilä of the University of Turku in Finland has already been closely scrutinised by physicists from both the LIGO and Virgo detectors at the centre of the research.

Nättilä and his colleagues set about refining estimates for the size of neutron stars by calculating the x-ray radiation previously recorded from a low-mass binary neutron star dubbed 4U 1702-429. The radiation is produced by intense atomic explosions taking place on the surface of the star.

Using five x-ray bursts detected by the Rossi X-ray Timing Explorer, a NASA satellite launched in 1995, the scientists compared the data to state-of-the-art neutron star models and tracked the difference between real and predicted outcomes.

The result revealed that the radius of a neutron star is 12.4 kilometres, with a margin of error of only another 400 metres, plus or minus. This means the stars are at the lower end of earlier estimates.

The research also found that 4U 1702-429, and likely therefore all other similar stars, had a gravitational mass of 1.9 times that of the sun, with a margin of 0.3 masses. The scientists caution, however, that mass calculations using available models are the “hardest to constrain”.

With the Finnish study adding unprecedented accuracy to earlier research, work is already underway to use its findings to better investigate the complex and delicate physics of gravitational waves.

The research is published in the journal Astronomy and Astrophysics, and a full version is available on the pre-print server arXiv.

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Andrew Masterson is news editor of Cosmos.
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