Twinkle, Twinkle, Neutron Star

June 16, 2009

Neutron stars are mysterious beasts. Sure, astrophysicists know they're the result of a massive star compressing during a supernova and collapsing in on itself. And they know it retains most of its angular momentum in the process, and has an incredibly high surface gravity. But they don't know what exactly the surface of a neutron star is made of, although it's clear that iron plays a role -- our instruments have detected the telltale spectral signature of iron in emissions from these objects. Nor is it clear whether the iron is in gaseous form, thereby forming a sort of "atmosphere," or whether it forms an ultra-hard solid crust.

A couple of weeks ago, a paper appeared on the arXiv with an intriguing means of telling the difference. Two Spanish scientists at the Universidad Complutense in Madrid conclude that if the iron in a neutron star is solid, it will form a rare and unusual crystal that is perfectly smooth and would envelop the entire star. And they've devised a method to test this by studying the surface of neutron stars using x-ray crystallography.

The idea is to look for binary neutron star systems: one "dead," with an iron crust, the other an x-ray pulsar. X-ray emissions from the pulsar should hit the surface of its partner, and those rays should be diffracted and thus detectable by our terrestrial instruments. Assuming they can find these sorts of couplings -- roughly 5% of neutron stars belong to binary star systems -- scientists could learn a great deal more about the structure and behavior of neutron stars.

There could be yet another means of studying the structure of neutron stars: observing the frequency spectra of stellar oscillations, more commonly known as "starquakes." There is actually a subfield known as asteroseismology, although it specializes in ordinary stars. Neutron stars also have these sorts of seismic events, in which the stiff surface crust ruptures much like terrestrial (tectonic) earthquakes. It happens because as a neutron star ages, its rotation gradually slows down, and its shape becomes more spherical through a series of stellar quakes.

Stellar quakes also cause neutron stars to flare brightly temporarily with so-called x-ray oscillations. Astrophysicists think this is because after the quake, the equatorial radius is slightly smaller; neutron stars spin and thus have angular momentum, which must be conserved. So the extra energy is released as x-rays.

That's bad news for x-ray satellites, since they are momentarily blinded by the light. But it's good news for the x-ray photons themselves, which finally have sufficient energy to overcome the star's immense surface gravity (about 10<11> times that of Earth) and escape. Sometimes photons need their freedom, too -- it's a big, big universe out there.

Photo:  An artists's concept of the 2004 occurence in which a neutron star underwent a "star quake", causing it to flare brightly, temporarily blinding all x-ray satellites in orbit. Source: NASA. Public domain.

Comments

[ImaginaryUnit]

"it's good news for the x-ray photons themselves, which finally have sufficient energy to overcome the star's immense surface gravity"

As long as the neutron star is not a black hole, any photon (x-ray or not) will always be able to escape.

[IBY]

Bizarre stuff... Which is what I like. ^_^
Two questions, though... Doesn't iron form towards the center when fusion occurs, so why does it occur on the crust?
Secondly, what do you mean by a neutron star that is "dead" ?

[ImaginaryUnit]

@IBY : Fusion is something that would happen inside a normal star like our Sun. When the fusion dies out (due to old age) and becomes too weak to support the star's own mass, the star collapses into something much denser, perhaps a black hole, or perhaps a neutron star, which is something like a huge atomic core. At this stage, fusion as we know it in our Sun is no longer happening.

'Dead' here would mean passive, not radiating anymore, as opposed to the 'alive' pulsar, which is pulsing off huge amounts of radiation.