Fermi Gamma-ray Space Telescope

Polarized Comptonized Emission in Magnetar Flares

J. A. Barchas
M. G. Baring

Abstract:

Spectral investigation of magnetar flares is an active topic, and their intrinsic brightness leads to inferences of high Thomson opacity, so that it is expected that Comptonized emission is present. In regions of strong magnetic field, there are different Compton scattering cross sections for the two linear polarization modes. Different resultant opacities suggest disparate locales of the emission for the two modes. Broadband observations of magnetar flares such as those seen in SGR J1550-5418 by Fermi-GBM, Swift and other instruments are compatible with a spectral fit that consists of a sum of blackbodies, which can be physically interpreted as temperature gradients with respect to optical depth in the magnetosphere. Considering the polarization of emission provides a natural way to explain the summed blackbody fit. In order to study this further we are developing a spectral model of polarized radiation transfer due to Compton scattering in magnetic field. We use a Monte Carlo simulation to examine the spectrum and polarization of photons emerging from a region of prescribed magnetic field geometry and density of Compton scatterers. Results presented give the emergent photon intensity and polarization in a dipole magnetic field with scattering in the magnetic Thomson regime, anticipating future extensions to the full QED domain.