XMM-Newton captured detailed views of the X-ray emission from five pulsars, each of which was up to several million years old. "No other X-ray satellite can do this work. Only XMM-Newton is capable of observing details of their X-ray emission," says Becker. He and his collaborators found no evidence of surface emission, nor of polar hotspots, although they did see emission from the outwardly moving particles.
The lack of surface emission is no surprise. In the several million years since their birth these pulsars have cooled from billions of degrees to much less than 500 000 degrees Celsius, meaning that their surface-wide X-ray emission has faded from view.
However, the lack of the polar hotspots in old pulsars is a big surprise and shows that the heating of the polar surface regions by particle bombardment is not efficient enough to produce a significant thermal X-ray component. "In the case of three-million-year-old pulsar PSR B1929+10 the contribution from any heated polar region is less than seven percent of the total detected X-ray flux," says Becker.
It seems that the conventional view is not the only way to look at the problem. An alternative theory is that the heat trapped in the pulsar since its birth will be guided to the poles by the intense magnetic field within the pulsar. This is because the heat is carried on electrons, which are electrically charged and so will be directed by magnetic fields.
This means that the polar hot spots in younger pulsars are produced predominantly from heat within the pulsar, rather than from the collision of particles from outside the pulsar. They will therefore fade from view in the same way as the surface-wide emission. "This view is still under discussion but is very much supported by the new XMM-Newton observations," says Becker.
Nearly forty years since the discovery of pulsars, it seems that old pulsars still have new tricks to teach astronomers.