Resonant Cyclotron Scattering in Magnetars’ Emission

Abstract

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 (AXPs), and soft gamma repeaters (SGRs). 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. However, while the RCS model can alone reproduce the soft X-ray spectra of AXPs, the much harder spectra of SGRs below 10 keV, requires the addition of a power-law component (the latter being the same component responsible for their hard X-ray emission). 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 component into account in our modeling not to overlook their contribution in the ∼4-10 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 n GJ , 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.

Force-free twisted magnetospheres of neutron stars

“…These models establish a new basis for generalizing the study of radiation transfer in neutron star magnetospheres, as already extensively done with the self-similar models (Rea et al 2008;Fernández & Thompson 2007;Zane et al 2009). In particular, our approach provides a more general framework for studying how the geometry of the magnetic field and currents affects the output X-ray spectrum of a magnetar.…”

X-Ray Observations of Magnetar SGR 0501+4516 from Outburst to Quiescence

“…In addition to the BB+PL and 2BB+PL models, we also tried the resonant cyclotron scattering (RCS) (Rea et al 2008) and the 3D surface thermal emission and magnetospheric scattering (STEM3D) models (Weng & Gögüş 2015) but the fits converged to the boundary of the parameter space. Therefore, we do not believe that the results are physical.…”

Surface emission from neutron stars and implications for the physics of their interiors

“…Hydrogen atmosphere models with high magnetic field strengths naturally generate such broad spectra (see Section 2.3). Reprocessing of the surface emission by mildly relativistic charges in the magnetosphere further broadens the spectrum and suppresses the equivalent widths of cyclotron features (Gavriil & Lyutikov 2006;Güver et al 2007Güver et al , 2008Rea et al 2008). Detailed comparisons of these models to observations have been successful and provide a spectroscopic measurement of the neutron star magnetic field strength (see Section 6).…”