Spectra and Polarization from Comptonized Emission in Magnetar Flares
Abstract
Magnetar flares exhibit extremely large luminosities, usually far in excess of the non-magnetic Eddington limit for neutron stars. The energetics of these outbursts implies large Thomson optical depths. Their spectra in the 2-200 keV band drive the expectation that Comptonized emission is present. The spectra observed from the storm of bursts in SGR J1550-5418 in January 2009 are compatible with a sum of two blackbodies (or more), which can be physically interpreted as hot regions in disparate locations in the neutron star magnetosphere. In strong magnetic fields, there are different Compton scattering cross sections for the two linear polarization modes, and furthermore these become resonant at the cyclotron frequency. These generate different opacities and different locales for the photospheres associated with the two modes. Considering the polarization of emission provides a natural way to explain the dual blackbody spectral fits. In order to address these observations, we are developing a spectral model of polarized radiation transfer due to Compton scattering in superstrong magnetic fields. We use a Monte Carlo simulation to examine the spectrum, polarization and anisotropy of photons emerging from a region of prescribed magnetic field and electron density. The initial results indicate that the emergent spectrum is often dominated by the extraordinary polarization mode near and below the cyclotron frequency for different values of the magnetic field strength and optical depth in the Thomson regime.
- Publication:
-
American Astronomical Society Meeting Abstracts #225
- Pub Date:
- January 2015
- Bibcode:
- 2015AAS...22521402B