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...

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