Resonant Cyclotron Scattering in Magnetars’ Emission

We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X-ray data of ten magnetars, including canonical and transient anomalous X-ray pulsars, and soft gamma repeaters. In this scenario, non-thermal magnetar spectra in the soft X-rays (i.e. below ∼ 10 keV) result from resonant cyclotron scattering of the thermal surface emission by hot magnetospheric plasma. We find that this model can successfully account for the soft X-ray emission of magnetars, while using the same number of free parameters than the commonly used empirical blackbody plus power-law model. Although this model in its present form does not explain the hard X-ray emission of a few of these sources, we took this further components into account in our modeling not to overlook their contribution in the ∼410 keV band. We find that the entire class of sources is characterized by magnetospheric plasma with a density which, at resonant radius, is about 3 orders of magnitudes higher than nGJ , the Goldreich-Julian electron density. The inferred values of the intervening hydrogen column densities, are also in better agreement with more recent estimates inferred from the fit of single X-ray edges. For the entire sample of observations, we find indications for a correlation between the scattering depth and the electron thermal velocity, and the field strength. Moreover, in most transient anomalous X-ray pulsars the outburst state is characterized by a relatively high surface temperature which cools down during the decay, while the properties of the magnetospheric electrons vary in a different way from source to source. Although the treatment of the magnetospheric scattering used here is only approximated, its successful application to all magnetars we considered shows that the RCS model is capable to catch the main features of the spectra observed below ∼ 10 keV. Subject headings: radiation mechanisms: non-thermal — stars: magnetic fields — stars: neutron — X-rays: individual (4U 0142+614 1RXSJ1708-4009, 1E 1841-045 , 1E 2259+586 , 1E1048-5937 , XTEJ1810-197 , 1E 1547.0-5408, CXOUJ1647-4552, SGR1806-20 , SGR1900+14)