Super-Eddington Radiation Transfer in Soft Gamma Repeaters

Abstract

Bursts from soft gamma repeaters have been shown to be super-Eddington by a factor of 1000 and have been persuasively associated with compact objects. Here, a model of super-Eddington radiation transfer on the surface of a strongly magnetic ($\geq 10^{13}$ gauss) neutron star is studied and related to the observational constraints on soft gamma repeaters. In strong magnetic fields, the cross-section to electron scattering is strongly suppressed in one polarization state, so super-Eddington fluxes can be radiated while the plasma remains in hydrostatic equilibrium. The model offers a somewhat natural explanation for the observation of similarity between spectra from bursts of varying intensity. The radiation produced in the model is found to be linearly polarized to about 1 part in 1000 in a direction determined by the local magnetic field, and the large intensity variations between bursts are understood as a change in the radiating area on the source. Therefore, the polarization may vary as a function of burst intensity, since the complex structure of the magnetic field may be more apparent for larger radiating areas. It is shown that for radiation transfer calculations in this limit of super-strong magnetic fields it is sufficient to solve the radiation transfer equations for the low opacity state rather than the coupled equations for both. With this approximation, standard stellar atmosphere techniques are utilized to calculate the model energy spectrum.