X-ray spectra from magnetar candidates. I. Monte Carlo simulations in the non-relativistic regime

Abstract

The anomalous X-ray pulsars and soft gamma-repeaters are peculiar high-energy sources believed to host a magnetar, i.e. an ultra-magnetized neutron star. Their persistent, soft X-ray emission (~1-10 keV)is usually modeled by the superposition of a blackbody and a power-law tail. It has been suggested that this spectrum forms as the thermal photons emitted by the star surface traverse the magnetosphere. Magnetar magnetospheres are likely different from those of ordinary radio-pulsars, since the external magnetic field may acquire a toroidal component as a consequence of the deformation of the star crust induced by the super-strong interior field. In turn, the magnetosphere will be permeated by currents that can boost primary photons through repeated scatterings. Here we present 3D Monte Carlo simulations of photon propagation in a twisted magnetosphere. Our model is based on a simplified treatment of the charge carriers velocity distribution which, however, accounts for the particle collective motion, in addition to the thermal one. Present treatment is restricted to conservative (Thomson) scattering in the electron rest frame. The code, nonetheless, is completely general and inclusion of the relativistic QED resonant cross section, which is required in the modeling of the hard (~20-200 keV) spectral tails observed in the magnetar candidates, is under way. The properties of emerging spectra have been assessed under different conditions, by exploring the model parameter space, including effects arising from the viewing geometry. Monte Carlo runs have been collected into a spectral archive. Two tabulated XSPEC spectral models, with and without viewing angles, have been produced and applied to the 0.1-10 keV XMM-Newton EPIC-pn spectrum of the AXP CXOU J1647-4552.

37 pages, 14 figures, 1 table; corrected typos; accepted for publication in MNRAS

References in corpus (5)

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• Exciting the Magnetosphere of the Magnetar CXOU J164710.2-455216 in Westerlund 1
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Cited by in corpus (7)

• The strongest cosmic magnets: Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars
• Untwisting magnetospheres of neutron stars
• Resonant cyclotron scattering in magnetars' emission
• X-ray spectra from magnetar candidates. II Resonant cross sections for electron-photon scattering in the relativistic regime
• From outburst to quiescence: the decay of the transient Anomalous X-ray Pulsar XTE J1810-197
• Topology of magnetars external field. I. Axially symmetric fields
• XMM-Newton discovery of 2.6 s pulsations in the soft gamma-ray repeater SGR 1627-41