A phase-dependent view of cyclotron lines from model accretion mounds on neutron stars

Abstract

In this paper we make a phase-dependent study of the effect of the distortion of the local magnetic field due to confinement of the accreted matter in X-ray pulsars on the cyclotron spectra emitted from the hotspot. We have numerically solved the Grad–Shafranov equation for axisymmetric static magnetohydrodynamic (MHD) equilibria of the matter confined at the polar cap of neutron stars. From our solution, we model the cyclotron spectra that will be emitted from the region, using a simple prescription and integrating over the entire mound. Radiative transfer through the accretion column overlying the mound may significantly modify the spectra in comparison to those presented here. However, we ignore this in the present paper in order to expose the effects directly attributable to the mound itself. We perform a spin phase-dependent analysis of the spectra to study the effect of the viewing geometry.