A simple mechanism for the anti-glitch observed in AXP 1E 2259+586

Abstract

In this letter, we develop a simple internal mechanism that can account for the recent 'antiglitch' observed for AXP 1E 2259+586 (Δν/ν ≳ 10-7). We propose that the cumulative decay of the internal toroidal magnetic field component will eventually become large enough to turn an originally stable prolate stellar configuration into an unstable one. The subsequent rearrangement of the stellar structure will result in a sudden spin-down of the whole star. We present order-of-magnitude calculations to give confidence to this scenario, using a simple, but physically reasonable, analytical stellar model. We estimate the energy released by the proposed mechanism and show that it is in agreement with the observations. Based on this model, we predict that to achieve the observed sudden spin-down, a minimum magnetar-like value for the magnetic field strength is needed. Consequently, we do not expect this kind of anti-glitch activity to occur in normal pulsars.