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On the Spectrum and Polarization of Magnetar Flare Emission

1
Department of Physics and Astronomy, University of Padova, via Marzolo 8, I-35131 Padova, Italy
2
Department of Mathematics and Physics, University of Roma 3, via della Vasca Navale 84, I-00146 Roma, Italy
3
Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Surrey RH5 6NT, UK
*
Author to whom correspondence should be addressed.
Galaxies 2018, 6(1), 35; https://doi.org/10.3390/galaxies6010035
Received: 31 January 2018 / Revised: 28 February 2018 / Accepted: 28 February 2018 / Published: 12 March 2018
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
Bursts and flares are among the distinctive observational manifestations of magnetars, isolated neutron stars endowed with an ultra-strong magnetic field ( B 10 14 10 15 G). It is believed that these events arise in a hot electron-positron plasma, injected in the magnetosphere, due to a magnetic field instability, which remains trapped within the closed magnetic field lines (the “trapped-fireball” model). We have developed a simple radiative transfer model to simulate magnetar flare emission in the case of a steady trapped fireball. After dividing the fireball surface in a number of plane-parallel slabs, the local spectral and polarization properties are obtained integrating the radiative transfer equations for the two normal modes. We assume that magnetic Thomson scattering is the dominant source of opacity, and neglect contributions from second-order radiative processes, although the presence of double-Compton scattering is accounted for in establishing local thermal equilibrium in the fireball atmospheric layers. The spectra we obtained in the 1–100 keV energy range are in broad agreement with those of available observations. The large degree of polarization (≳80%) predicted by our model should be easily detectable by new-generation X-ray polarimeters, like IXPE, XIPE and eXTP, allowing one to confirm the model predictions.
Keywords: polarization; radiative transfer; scattering; techniques: polarimetric; stars: magnetars; X-rays: bursts polarization; radiative transfer; scattering; techniques: polarimetric; stars: magnetars; X-rays: bursts
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Taverna, R.; Turolla, R. On the Spectrum and Polarization of Magnetar Flare Emission. Galaxies 2018, 6, 35.

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