Anomalous X‐Ray Pulsars and Soft Gamma‐Ray Repeaters: Spectral Fits and the Magnetar Model

Abstract

The energy source powering the X-ray emission from anomalousX-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) is stilluncertain. In one scenario, the presence of an ultramagnetized neutronstar, or ``magnetar,'' with B~=1014-1015 G isinvoked. To investigate this hypothesis, we have analyzed archivalASCA data for several known AXPs and SGRs and fitted them with amodel in which all or part of the X-ray flux originates as thermalemission from a magnetar. Our magnetar spectral model includes theeffects of the anisotropy of the heat flow through an ultramagnetizedneutron star envelope, reprocessing by a light-element atmosphere,and general relativistic corrections to the observed spectrum. Weobtain good fits to the data with radii for the emitting areas thatare generally consistent with those expected for neutron stars, incontrast to blackbody (BB) fits, which imply much smaller radii.Furthermore, the inclusion of atmospheric effects results in inferredtemperatures lower than those implied by BB fits, but still too highto be accounted for by thermal cooling alone. An extra source ofheating (possibly resulting from magnetic field decay) is needed.Despite the harder tail in the spectrum produced by reprocessingof the outgoing flux through the atmosphere, spectral fits stillrequire a considerable fraction of the flux to be in a power-lawcomponent, probably produced in the magnetosphere of the star.