Electrodynamics of Magnetars: Implications for the Persistent X‐Ray Emission and Spin‐down of the Soft Gamma Repeaters and Anomalous X‐Ray Pulsars

Abstract

(ABBREVIATED) We consider the structure of neutron star magnetospheres\nthreaded by large-scale electrical currents, and the effect of resonant Compton\nscattering by the charge carriers (both electrons and ions) on the emergent\nX-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs,\nthese currents are maintained by magnetic stresses acting deep inside the star.\nWe construct self-similar, force-free equilibria of the current-carrying\nmagnetosphere with a power-law dependence of magnetic field on radius, B ~\nr^(-2-p), and show that a large-scale twist softens the radial dependence to p\n< 1. The spindown torque acting on the star is thereby increased in comparison\nwith a vacuum dipole. We comment on the strength of the surface magnetic field\nin the SGR and AXP sources, and the implications of this model for the narrow\nmeasured distribution of spin periods. A magnetosphere with a strong twist,\nB_\\phi/B_\\theta = O(1) at the equator, has an optical depth ~ 1 to resonant\ncyclotron scattering, independent of frequency (radius), surface magnetic field\nstrength, or charge/mass ratio of the scattering charge. When electrons and\nions supply the current, the stellar surface is also heated by the impacting\ncharges at a rate comparable to the observed X-ray output of the SGR and AXP\nsources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux\nat the ion and electron cyclotron resonances will significantly modify the\nemerging pulse profile and, through the Doppler effect, generate a non-thermal\ntail to the X-ray spectrum. The sudden change in the pulse profile of SGR\n1900+14 after the 27 August 1998 giant flare is related to an enhanced optical\ndepth to electron cyclotron scattering, resulting from a sudden twist imparted\nto the external magnetic field.\n