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Resonant cyclotron scattering in magnetars' emission

Rea, N; Zane, S; Turolla, R; Lyutikov, M; Götz, D; (2008) Resonant cyclotron scattering in magnetars' emission. Astrophysical Journal , 686 (2) pp. 1245-1260. 10.1086/591264.

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Abstract

We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X-ray data of 10 magnetars, including canonical and transient anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs). In this scenario, nonthermal 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 as in 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 require 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 emis sion (i.e., above ∼20 keV) of a few of these sources, we took this further component into account in our modeling not to overlook its 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 magnitude higher than the Goldreich-Julian electron density. The inferred values of the intervening hydrogen column densities are also in better agreement with more recent estimates. Although the treatment of the magnetospheric scattering used here is only approximated, its successful application to all magnetars shows that the RCS model is capable of catching the main features of the spectra observed below ∼10 keV. © 2008. The American Astronomical Society. All rights reserved.

Type: Article
Title: Resonant cyclotron scattering in magnetars' emission
DOI: 10.1086/591264
UCL classification: UCL > School of BEAMS
UCL > School of BEAMS > Faculty of Maths and Physical Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/179401
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