Computation of Neutron Star Surface Emission Spectra for Arbitrary Magnetic Field Directions without Diffusion Approximation

Abstract

To derive physical properties of the neutron star surface with observed spectra, a realistic model spectrum of neutron star surface emission is essential. Limited by computing resources, a full computation of the radiative transfer equations without the diffusion approximation has been conducted up to date only for the case of local magnetic fields being perpendicular to the stellar surface. In this paper we report the full-computation result for an arbitrary field direction. For comparison we also compute the radiative transfer equation using the diffusion approximation. For a given effective temperature, the computed spectrum with the diffusion approximation is always softer than that of a full computation at a non-negligible level. It leads to an over-estimate of the effective temperature if the diffusion approximation spectrum is employed in the spectral fitting. Other characteristics for different magnetic field orientations, such as the beaming pattern of the two polarization modes and the structure of the atmosphere, are also discussed.