An anti-glitch in a magnetar

Abstract

Magnetars sometimes exhibit mysterious ‘glitches’, during which angular momentum is transferred between the star’s outer and inner crusts, and involving a sudden spin-up of the star; here X-ray timing observations reveal a sudden spin-down, or ‘anti-glitch’ in a magnetar. Hundreds of glitches observed in the emission of radio pulsars and magnetars — strongly magnetic neutron stars emitting X- and γ-rays — have involved a sudden spin-up, or increase in the surface angular velocity. Glitches are thought to arise when angular momentum is transferred between the solid outer crust and the superfluid component of the inner crust. This paper reports the first observation of an 'anti-glitch', a sudden spin-down event, in the magnetar 1E 2259+586. The event coincided with an X-ray flare and X-ray bursts similar to those seen during some previous magnetar spin-up glitches, suggesting an origin in the stellar interior rather than the magnetosphere. Current models of neutron star spin-down do not predict such behaviour. Magnetars are neutron stars with X-ray and soft γ-ray outbursts thought to be powered by intense internal magnetic fields1. Like conventional neutron stars in the form of radio pulsars, magnetars exhibit ‘glitches’ during which angular momentum is believed to be transferred between the solid outer crust and the superfluid component of the inner crust2,3,4. The several hundred observed glitches in radio pulsars5,6 and magnetars7 have involved a sudden spin-up (increase in the angular velocity) of the star, presumably because the interior superfluid was rotating faster than the crust. Here we report X-ray timing observations of the magnetar 1E 2259+586 (ref. 8), which exhibited a clear ‘anti-glitch’—a sudden spin-down. We show that this event, like some previous magnetar spin-up glitches9, was accompanied by multiple X-ray radiative changes and a significant spin-down rate change. Such behaviour is not predicted by models of neutron star spin-down and, if of internal origin, is suggestive of differential rotation in the magnetar, supporting the need for a rethinking of glitch theory for all neutron stars10,11.