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For the first time astronomers have seen a rapidly spinning neutron star slow down, a phenomenon that has been labelled an "anti-glitch".
Neutron stars are the densest, most massive objects in the Universe after black holes -- despite diameters that are often only a few kilometres, they can be millions of times the mass of the Sun. A bowlful of neutron star would be as massive as a planet.
It's so dense that it's actually denser than a standard atomic nucleus, the atoms are mushed together so tightly.
They spin very quickly due to the conservation of momentum.
Their increase in mass as they form, combined with their dramatic decrease in size, makes them spin faster and faster until they reach a set speed. Over the course of millennia a neutron star will slow down because it's losing energy, but that rate of slowdown is extremely slow and predictable, on the order of fractions of a second for every thousand years.
So it was very surprising for astronomers to spot 1E 2259+586 -- a neutron star 10,000 light years from Earth with an extremely strong magnetic field known as a magnetar -- undergo a relatively rapid slowdown. It had been spinning at roughly one revolution every seven seconds throughout a year of observations, but on 28 April 2012 a team from McGill University using Nasa's Swift X-ray telescope realised that it had unexpectedly slowed down by 2.2 millionths of a second more than it should have. "I looked at the data and was shocked -- the neutron star had suddenly slowed down," said astrophysicist Robert Archibald, lead author of the paper published in Nature. "These stars are not supposed to behave this way."
It's actually quite normal to see neutron stars undergo sudden changes in rotational speed, but until now they've only ever been seen to suddenly shift a gear and rotate faster. These events -- called "glitches" -- happen when there's a sudden ejection of mass by the star into space, for reasons that are still unclear. It's theorised that neutron stars have a "crust" of ions and electrons that becomes rigid over time, eventually cracking apart like the tectonic plates on Earth and letting high-energy particles from the star's interior escape into space.
Conservation of momentum comes into play again, and the star rotates faster. But what happened to 1E 2259+586 was the opposite -- it started moving slower. It's an anti-glitch, the first one ever to be observed, and it reinforces the suspicion that our existing theories of how glitches occur are inadequate.
The most obvious reason for a decrease in speed, bearing the principle of conservation of momentum in mind, would then be that 1E 2259+586 had gained some mass -- perhaps from a large, unseen planet wandering into its path and getting sucked in. Due to the extreme gravity near the surface of a neutron star any such planet would (we expect) break up quickly into a belt of material as it fell in, and if that happened, you'd expect to see a corresponding strong increase in X-ray emissions.
That happened, but only briefly -- for 36 milliseconds on 21 April. The astronomers believe this to be the cause of, or at the very least is related to, the star's slowdown, but exactly why is unknown. It's unlikely that the event of a planet getting sucked into the star would be over so quickly, as accretion disks can last for weeks before evaporating.
While something clearly happened, the burst of X-rays further challenges the dominant neutron star theory. It's impossible for us to recreate the conditions inside neutron stars here on Earth, so remote observations like this are the only way to figure out their bizarre physical properties.
This article was originally published by WIRED UK
