Abstract
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.
Highlights
IntroductionMagnetars are a particular kind of isolated neutron stars (NSs), observationally identified with
Magnetars are a particular kind of isolated neutron stars (NSs), observationally identified withSoft Gamma Repeaters (SGRs) and Anomalous X-ray pulsars (AXPs)
The intensities calculated in the radiative transfer code and processed in the ray-tracer are used to obtain the simulated spectra and the polarization observables as observed at infinity
Summary
Magnetars are a particular kind of isolated neutron stars (NSs), observationally identified with. According to the magnetar model, first developed by [3], the huge internal magnetic field of magnetars is believed to develop a large toroidal component, which exerts a strong magnetic stress on the conductive star crust. Galaxies 2018, 6, 35 the sudden re-arrangements of the external field lines that follow the crustal displacements triggered by the strong internal field. This injects in the magnetosphere an Alfvén pulse that dissipates into an electron-positron pair plasma, which remains magnetically confined within the closed field lines (the “trapped fireball”).
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