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Modelling the magnetars quiescent spectra

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Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are peculiar X-ray sources which are believed to be magnetars: ultra-magnetized neutron stars with surface field in excess of 1E14 G, i.e. well above the QED threshold. Spectral analysis is an important tool in magnetar astrophysics since it can provide key information on the emission mechanisms. The first attempts at modelling the persistent (i.e. outside bursts) soft X-ray ( 0.3-0.6 keV) plus a power-law (photon index 2-4) could successfully reproduce the observed emission. Moreover, recent INTEGRAL observations have shown that, while in quiescence, magnetars emit substantial persistent radiation also at higher energies, up to a few hundreds of keV. Their high energy spectra are tipically modelled by a further power law which in some cases has been proved to exhibit a strong dependence on the spin phase. However, a convincing physical interpretation of the various spectral components is still missing and our knowledge of the broadband emission is severely hampered by the impossibility, so far, to carry out simultaneous observations in the whole energy range. In this talk I will focus on the interpretation of magnetar spectral properties during quiescence. I will summarise the present status of the art and our first attempts to model the broadband persistent emission of magnetars within a self consistent, physical scenario. I will then present the predictions of a synthetic model that we calculated with a new magnetic montecarlo 3D radiative code. Our code accounts for resonant cyclotron upscattering of soft thermal photons (emitted by the star surface), by a population of relativistic electrons threated in the magnetosphere. Polarization and QED effects are consistently accounted for, as well different configurations for the magnetosphere. I will discuss the predicted spectral and polarization properties in the 0.1-100 keV range, and the application to existing magnetars data.

This talk is part of the New Results in X-ray Astronomy 2009 series.

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