Magnetospheric Structure and Non-Thermal Emission of AXPs and SGRs

Abstract

In the framework of the magnetar model for the Soft Gamma Repeaters and Anomalous X-ray Pulsars, we consider the structure of neutron star magnetospheres threaded by large-scale electrical currents. We construct self-similar, force-free equilibria under the assumption of axisymmetry and a power law dependence of magnetic field on radius, ${\bf B} \propto r^{-(2+p)}$. A large-scale twist of the field lines softens the radial dependence to $p < 1$, thereby accelerating the spindown torque with respect to a vacuum dipole. A magnetosphere with a strong twist ($B_ϕ/B_θ= O(1)$ at the equator) has an optical depth $\sim 1$ to resonant cyclotron scattering, independent of frequency (radius), surface magnetic field strength, or the charge/mass ratio of the scattering charge. We investigate the effects of the resonant Compton scattering by the charge carriers (both electrons and ions) on the emergent X-ray spectra and pulse profiles.