Resonant cyclotron scattering in magnetars' emission
Faculty of Science (FNWI)
Anton Pannekoek Institute for Astronomy (API)
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 emission (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.
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