A neutron star suddenly slows its spin

A neutron star is a compact ball of matter in extremis—a sunlike mass stuffed into a sphere about 20 km across—left behind by a supernova explosion. Set spinning by the explosion, such a star is thought to consist of a kilometer-thick crust of electrons and nuclei encasing a rich superfluid. Thanks to their magnetic fields, neutron stars emit dipole radiation and accelerate charged particles outward through their crust; thus they are always losing energy and angular velocity. Curiously, that process is occasionally interrupted by “glitches” in which a star abruptly spins up by a small amount. Those events, according to models, may be attributable to the faster-moving superfluid exerting enough stress to sometimes fracture the crust and transfer angular momentum. While monitoring a hypermagnetized neutron star known as a magnetar on 28 April 2012 using NASA’s Swift observatory, astronomers noticed something unexpected: an “anti-glitch,” the abrupt 2-µs slowing of the spin from the star’s roughly 7-s period. Just a week earlier, the same magnetar had produced a 36-ms x-ray burst, a telltale sign of events in the star that led to the anti-glitch. But in addition to the sudden slowing, Swift recorded an extended period after the anti-glitch when the magnetar’s spin rate slowed further still. No theory accounts for the observations, but the researchers suspect two possible mechanisms: The transfer of angular momentum from some part of the superfluid rotating more slowly than the crust, or the gradual untwisting of twisted magnetic field lines. (R. F. Archibald et al., Nature 497, 591, 2013, doi:10.1038/nature12159 .)