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Astronomers spy hot spots on neutron stars

No, not that kind of hot spot...

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European astronomers have seen, for the first time, rotating hotspots on the surfaces of three (relatively) nearby neutron stars. The discovery sheds new light on the thermal geography of these stars - the incredibly hot and dense remnants of supernova explosions.

The pictures, taken with the XMM-Newton space telescope, reveal surface features ranging from the size of a football field to that of a golf course, on bodies hundreds, even thousands of light-years away.

An X-ray image of the neutron star PSR B0656-14. Image: ESA

Neutron stars are what you get when a star-proper explodes in a type II supernova. As the core of a massive star collapses, the pull of gravity is sufficiently strong to force protons and electrons to combine and form neutrons. Typically, a neutron star will have the mass of one and a half suns compressed into a spherical volume just 10-15km in diameter.

As well as being incredibly dense, they rotate very fast, and are blindingly hot - cooling down over hundreds of thousands of years to reach a just-scorching million or so degrees.

Using the XMM-Newton, astronomers divided the surface of each of the three target stars into ten co-called wedges, and measured the temperature at the surface of each wedge. This way, they could track the hot spot as the star rotated.

Scientists have predicted the existence of these hot spots - regions where the electromagnetic energy emitted by the star is funneled back down to the surface. Theorists hypothesized that the influx of energy would reheat a local area on the surface so that it was much hotter than its cooling surroundings.

The research team suspect that the hot spots are linked to polar regions, and are in some way similar to the Aurorae on planets with magnetic fields. In both cases, the team think the phenomenon is caused by charged particles being trapped in magnetic field lines and carried towards the surface.

Patrizia Caraveo, of the Istituto Nazionale di Astrofisica (IASF) in Italy commented: "This result is a first, and a key to understand the internal structure, the dominant role of the magnetic field treading the star interior and its magnetosphere, and the complex phenomenology of neutron stars."

The researchers are still trying to account for the difference in size of the hot regions on each of the stars. ®

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