  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.
