Numerical study of plasma loading of twisted flux tubes in magnetar magnetospheres*

The plasma dynamics in neutron star magnetospheres produce various radiative phenomena such as X-ray emission and magnetar bursts. Magnetars are strongly magnetized but slowly rotating neutron stars. The slow rotation alone cannot release energies corresponding to the observed high-energy emission. As an alternative, we explore the twisting of flux tubes induced locally by internal motions of the crust. We perform radiative particle-in-cell simulations using a local setup of a straight flux tube starting from a vacuum, mimicking the dynamics of twisting magnetar flux tubes. Magnetic field lines are anchored to conducting plates with charge-supplying 'atmospheres' at the two ends of the tube. Shear motion on one conductor twists the flux tube, leading to parallel electric fields that extract and energize atmospheric particles. This work provides the non-radiative equilibrium state both analytically and numerically: large-scale electric fields form a so-called 'double layer' structure and gradually dissipate magnetic energy. When an ad-hoc pair-production is prescribed, fast particles can efficiently pair-produce, changing the plasma dynamics and slowing down the dissipation of the system. The results obtained in this idealized setup guide the investigation of plasma loading in a dipole magnetic field with realistic pair production processes for explaining the persistent magnetar X-ray emission.