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Astronomers Nathalie Degenaar and Rudy Wijnands of the University of Amsterdam (UvA) have observed the warming of the neutron star in an X-ray binary that was discovered in October 2010.

Astronomers Nathalie Degenaar and Rudy Wijnands of the University of Amsterdam (UvA) have observed the warming of the neutron star in an X-ray binary that was discovered in October 2010. In this way, they could take a look at the interior of such an extreme object. The researchers have described their results in two articles in the British science journal Monthly Notices of the Royal Astronomical Society (MNRAS).

In X-ray binaries, a neutron star and a companion star rotate around each other. Neutron stars are 1.5 times as heavy as the sun, but have a diameter of no more than 25 km. A teaspoon of neutron star material weighs more than 500 million tonnes. Due to the incredibly high density, they are interesting objects for investigating matter under extreme conditions. The companion transfers matter to the neutron star, creating X-ray emissions. The swallowing of matter generates energy (heat) that is stored in the neutron star. When the material transfer stops, the neutron star radiates the heat from its surface. Such heat can be measured with sensitive X-ray satellites.

Heating due to material transfer

After last year’s discovery of X-ray binary IGR J17480-2446 in the globular cluster Terzan 5, Degenaar and Wijnands looked through the archives of X-ray satellite Chandra to investigate the heat radiation of the new neutron star. Their analysis showed that the star had a relatively low temperature before the accretion phase began. They speculated that this temperature would probably rise once the transfer of the matter to the neutron star by the companion was stopped. In January 2011, the X-ray brightness of IGR J17480-2446 seemed to have become much lower, indicating that the transfer had been stopped. New Chandra observations ( in February 2011) showed that the neutron star was four times brighter (about 1.5 times as hot) than had previously been established based on the archival data. The conclusion of Degenaar and Wijnands is that the neutron star is indeed heated by the material transfer.

The astronomers expect the neutron star will now cool to its base level, and in order to study this more accurately a new Chandra measurement will be carried out in early May. This type of research has previously been conducted for a small group of X-ray binaries that sometimes swallow matter for years or decades and therefore heat up greatly. Wijnands has conducted pioneering work in this field. But never before has this been studied in a 'normal' X-ray binary, such as the newly discovered source, that only emits bright X-rays for a few weeks. With this discovery, research into the heating and cooling of neutron stars has taken a new direction. There are numerous models to describe how matter behaves under these extreme conditions. The astronomers can now see how quickly or slowly the star cools and thus exclude a number of models.