ESA Satellite puts new spin on pulsar's hotspots
Data from the European Space Agency's (ESA) X-ray satellite XMM-Newton shows that the polar hotspots observed on pulsars may be generated by internal rather than external energy sources, as was previously thought. Pulsars are the strongly magnetised, spinning cores of dead stars. Discovered almost 40 years ago, they are formed when massive stars use up their nuclear energy supply and collapse under their own weight. Although they are just 20 km across, their mass is approximately 1.4 times that of our Sun. Newly formed pulsars have temperatures in excess of a billion degrees, but they cool down gradually over time. Observations with earlier X-ray satellites revealed that the X-rays from cooling pulsars come from three sources: the surface of the pulsar, charged particles travelling outwards along the pulsar's magnetic field lines and hotspots at the polar regions of the pulsar. Until now it was thought that the hotspots at the poles arose when highly energetic charged particles slammed into the surface of the poles from the magnetosphere, guided there by the pulsar's magnetic field. In this latest study, researchers from the Max-Planck Institute for Extraterrestrial Physics in Germany used the XMM-Newton observatory to look at five pulsars which are several million years old and so have cooled down considerably. As the surface had cooled to below 500,000°C, the lack of X-rays coming from the surface was not unexpected. However, they were surprised to see that although there were still intensive X-ray emissions from outwardly moving particles in the magnetic field, there was no evidence of X-rays coming from the pulsars' poles. This implies that in old pulsars, the heating of the polar regions by particle bombardment is not enough to produce X-rays. Now the astronomers offer an alternative theory as to how the hotspots arise. They believe that the heat trapped within the pulsar at its birth is guided to the poles by the intense magnetic field within the pulsar. This means that the polar hot spots in younger pulsars are produced from heat within the pulsar, rather than from the collision of particles from outside the pulsar, and as it cools down over time, the hotspots fade. 'This view is still under discussion, but is very much supported by the new XMM-Newton observations,' said Werner Becker of the Max-Planck Institute.