Physical mechanisms for the variable spin-down and light curve of SGR 1900+14

We consider the physical implications of the rapid spin-down of soft gamma repeater SGR 1900 + 14 reported by Woods and colleagues in 1999. During an 80 day interval between 1998 June and the large outburst on 1998 August 27, the mean spin-down rate increased by a factor of 2.3, resulting in a positive period offset of DeltaP/P = 10(-4). A radiation-hydrodynamical outflow associated with the August 27 event could impart the required torque, but only if the dipole magnetic field is stronger than similar to 10(14) G and the outflow lasts longer and/or is more energetic than the observed X-ray flare. A positive period increment is also a natural consequence of a gradual, plastic deformation of the neutron star crust by an intense magnetic field, which forces the neutron superfluid to rotate more slowly than the crust. Sudden unpinning of the neutron vortex lines during the August 27 event would then induce a glitch opposite in sign to those observed in young pulsars, but of a much larger magnitude as a result of the slower rotation. The change in the persistent X-ray light curve following the August 27 event is ascribed to continued particle heating in the active region of that outburst. The enhanced X-ray output can be powered by a steady current flowing through the magnetosphere, induced by the twisting motion of the crust. The long-term rate of spin-down appears to be accelerated with respect to a simple magnetic dipole torque. Accelerated spin-down of a seismically active magnetar will occur when its persistent output of Alfven waves and particles exceeds its spin-down luminosity or if particle flows modulate the ratio of conduction to displacement currents in the outer magnetosphere. We suggest that SGRs experience some episodes of relative inactivity, with diminished P, and that such inactive magnetars are observed as anomalous X-ray pulsars (AXPs). The reappearance of persistent X-ray emission from SGR 1900+14 within one day of the August 27 event provides strong evidence that the persistent emission is not powered by accretion.