Peeking into the crust of a neutron star

The outer layer of a neutron star, its crust, covers about one-tenth of the total stellar radius and consists of ions, electrons and neutrons (see image). Studying the structure and composition of a neutron star crust is of interest because it plays an important role in the emission of gravitational waves and the evolution of the neutron star’s magnetic field.

When neutron stars reside in X-ray binaries, their crusts can be temporarily heated due to the accretion of matter. Once the accretion stops, the crust will cool again as it transports the gained heat towards the stellar core (where it is radiated away in the form of neutrino’s) and towards the surface (where the heat is lost in the form of thermal X-ray emission). Studying the heating and subsequent cooling of the crust of a neutron star carries unique information about its structure and gives insight into a variety of nuclear reaction processes.

We used the Chandra satellite to study the neutron star X-ray binary IGR 17480-2446, which is located in the globular cluster Terzan 5 and discovered in 2010 October. We observed the neutron star at different epochs after it had ended a 10-week episode of accretion. Our first observation revealed that the neutron star was hotter just after the accretion outburst than it had been before. In our subsequent observation the temperature had markedly decreased, although it was still higher than before the accretion activity. This suggests that the crust of the neutron star was heated during the accretion phase and is currently cooling down.

It is the first time that cooling of an accretion-heated neutron star crust has been observed for a neutron star with a “normal” accretion phase of a few weeks. Previous results concerned neutron stars that were heated for several years before the accretion stopped and the crust started to cool. By comparing the observed change in temperature with theoretical calculations, we found evidence for the presence of (strong) sources of heat in the outer layers of the crust. It remains a puzzle what should produce heat at such shallow layers. Further Chandra observations are planned to further investigate the temperature evolution of this neutron star.

Degenaar, Brown & Wijnands 2011, MNRAS Letters 418, L152: Evidence for crust cooling in the transiently accreting 11-Hz X-ray pulsar in the globular cluster Terzan 5

Paper link: ADS

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A schematic overview of the interior of a neutron star.

A schematic overview of the interior of a neutron star.