Twist-induced Magnetosphere Reconfiguration for Intermittent Pulsars

Abstract

We propose that the magnetosphere reconfiguration induced by magnetic twists in the closed field line region can account for the mode-switching of intermittent pulsars. We carefully investigate the properties of axisymmetric force-free pulsar magnetospheres with magnetic twists in closed field line region around the polar caps. The magnetosphere with twisted closed lines leads to enhanced spin-down rates. The enhancement in spin-down rate depends on the size of region with twisted closed lines. Typically, it is increased by a factor of $\sim2$, which is consistent with the intermittent pulsars' spin down behavior during the `off' and `on' states. We find there is a threshold of maximal twist angle $\Delta\phi_{\rm thres}\sim1$. The magnetosphere is stable only if the closed line twist angle is less than $\Delta\phi_{\rm thres}$. Beyond this value, the magnetosphere becomes unstable and gets untwisted. The spin-down rate would reduce to its off-state value. The quasi-periodicity in spin-down rate change can be explained by long-term activities in star's crust and the untwisting induced by MHD instability. The estimated duration time of on-state is about one week, consistent with observations. Due to the MHD instability, there exists an upper limit for the spin down ratio ($f\sim3$) between the on-state and the off-state, if the Y-point remains at the light cylinder.