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Nobody knows what the surface of neutron stars are made of but there’s no shortage of suggestions. We know that iron must play a role because we can see its characteristics absorption lines in the spectrum from these exotic objects.

But we can’t tell whether the iron forms a gaseous atmosphere, perhaps a meter or so thick above an ultra-hard crust, or whether the iron itself is solid.

Now Felipe Llanes-Estrada and Gaspar Moreno Navarro at the Universidad Complutense in Madrid Spain, say they know how to tell the difference. Here’s how:

If the iron is solid, then it ought to form an extraordinary crystal, perhaps a few dozen atomic layers thick, almost perfectly smooth and enveloping the entire star.

One way to examine crystalline solids is to use x-ray crystallography. The new idea from Llanes-Estrada and Navarro is to look for pairs of neutron stars in which one is an x-ray pulsar and the other is dead with an iron crust. Then x-rays from the pulsar should be diffracted from the surface of the dead star and detectable on Earth. The signature would consist of a main pulse from the pulsar followed by the reflection at wavelengths related by Bragg’s law of diffraction to the main pulse.

That’s a neat and potentially clean way to study the surface of neutron stars: if astronomers can find the right kind of pairs.

If they do, it’ll be a breath of fresh air for astrophysicists who have many ideas about the structure and behaviour of neutron stars but few ways to prune the dead wood from the theoretical undergrowth they’ve created.

Something for the team behind the Chandra X-ray telescope to get to work on, I’d say.

Ref: arxiv.org/abs/0905.4837: Bragg diffraction and the Iron crust of Neutron Stars

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