Constraints on Thermal Emission Models of Anomalous X‐Ray Pulsars

Thermal emission from the surface of an ultramagnetic neutron star isbelieved to contribute significantly to the soft X-ray flux of the AnomalousX-ray Pulsars. We compare the detailed predictions of models of the surfaceemission from a magnetar to the observed properties of AXPs. In particular, wefocus on the combination of their luminosities and energy-dependent pulsedfractions. We use the results of recent calculations for strongly magnetizedatmospheres to obtain the angle- and energy-dependence of the surface emission.We include in our calculations the effects of general relativistic photontransport and interstellar extinction. We find that the combination of thelarge pulsed fractions and the high luminosities of AXPs cannot be accountedfor by surface emission from a magnetar with two antipodal hot regions or atemperature distribution characteristic of a magnetic dipole. This result isrobust for reasonable neutron star radii, for the range of magnetic fieldstrengths inferred from the observed spin down rates, and for surfacetemperatures consistent with the spectral properties of AXPs. Models with asingle hot emitting region can reproduce the observations, provided that thedistance to one of the sources is ~30% less than the current best estimate, andallowing for systematic uncertainties in the spectral fit of a second source.Finally, the thermal emission models with antipodal emission geometry predict acharacteristic strong increase of the pulsed fraction with photon energy, whichis apparently inconsistent with the current data. The energy-dependence of thepulsed fraction in the models with one hot region shows a wider range ofbehavior and can be consistent with the existing data. Upcoming high-resolutionobservations with Chandra and XMM-Newton will provide a conclusive test.Comment: 25 preprint pages, 7 color figures, ApJ, in pres