Time-dependent force-free pulsar magnetospheres: axisymmetric and oblique rotators

Abstract

Magnetospheres of many astrophysical objects can be accurately described by the low-inertia (or "force-free") limit of MHD. We present a new numerical method for solution of equations of force-free relativistic MHD based on the finite-difference time-domain (FDTD) approach with a prescription for handling spontaneous formation of current sheets. We use this method to study the time-dependent evolution of pulsar magnetospheres in both aligned and oblique magnetic geometries. For the aligned rotator we confirm the general properties of the time-independent solution of Contopoulos et al. (1999). For the oblique rotator we present the 3D structure of the magnetosphere and compute, for the first time, the spindown power of pulsars as a function of inclination of the magnetic axis. We find the pulsar spindown luminosity to be L = (mu^2 Omega^4/c^3) (1+ sin^2(alpha)) for a star with the dipole moment "mu", rotation frequency "Omega", and magnetic inclination angle "alpha". We also discuss the effects of current sheet resistivity and reconnection on the structure and evolution of the magnetosphere.