Abstract
Bursts and flares are among the distinctive observational manifestations of magnetars, isolated neutron stars endowed with an ultra-strong magnetic field (B ≈ 1014–1015 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. We assume that magnetic Thomson scattering is the dominant source of opacity in the fireball medium, and neglect contributions from second-order radiative processes. 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 measured by new-generation X-ray polarimeters, like IXPE, XIPE and eXTP, allowing one to confirm the model predictions.
Highlights
Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are regarded as the observational manifestations of the same class of neutron stars (NSs), aka the magnetars, for which the long measured spin periods (P ≈ 2–12 s) and large period derivatives (P ≈ 10−13–10−10 ss−1) lead to a huge value of the dipole magnetic field, B ≈ 1014–1015 G, well above those of other NS classes
The neutron star geometry is described in a fixed frame (X, Y, Z) where the Z axis is chosen along the line-of-sight (LOS, unit vector ) and the X axis is in the plane made by and the star spin axis Ω
4 RESULTS 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
Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are regarded as the observational manifestations of the same class of neutron stars (NSs), aka the magnetars, for which the long measured spin periods (P ≈ 2–12 s) and large period derivatives (P ≈ 10−13–10−10 ss−1) lead to a huge value of the dipole magnetic field, B ≈ 1014–1015 G, well above those of other NS classes (see Turolla et al 2015; Mereghetti 2008, for reviews). An internal magnetic field instability induces large-amplitude oscillations in the magnetosphere, that convert, in turn, into a hot electron-positron plasma While part of this plasma quickly escapes outwards in the initial phases, producing the hard initial peak of giant flares, another part remains trapped within the closed magnetic field lines, resulting in an optically-thick, photon-pair fireball. Yang & Zhang (2015) have presented Montecarlo simulations to study the polarization properties of giant flare decay tail emission They asssumed that the pair plasma produced during the magnetar flare remains trapped within a set of closed (dipolar) magnetic field lines and solved the radiative transport in a geometrically-thin, surface layer of the fireball, where magnetic Thomson scattering is the only source of opacity.
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