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

The energy source powering the X-ray emission from anomalous X-ray pulsars(AXPs) and soft gamma-ray repeaters (SGRs) is still uncertain. In one scenario,the presence of an ultramagnetized neutron star, or ``magnetar'', with B on theorder of 10^{14} - 10^{15} G is invoked. To investigate this hypothesis, wehave analyzed archival ASCA data for several known AXPs and SGRs, and fittedthem with a model in which all or part of the X-ray flux originates as thermalemission from a magnetar. Our magnetar spectral model includes the effects ofthe anisotropy of the heat flow through an ultramagnetized neutron starenvelope, reprocessing by a light element atmosphere, and general relativisticcorrections to the observed spectrum. We obtain good fits to the data withradii for the emitting areas which are generally consistent with those expectedfor neutron stars, in contrast to blackbody (BB) fits, which imply much smallerradii. Furthermore, the inclusion of atmospheric effects results in inferredtemperatures which are lower than those implied by BB fits, but however stilltoo high to be accounted by thermal cooling alone. An extra source of heating(possibly due to magnetic field decay) is needed. Despite the harder tail inthe spectrum produced by reprocessing of the outgoing flux through theatmosphere, spectral fits still require a considerable fraction of the flux tobe in a power-law component.Comment: 14 pages, 2 tables, 1 figure, ApJ in press; note added to Table