Phase dependent view of Cyclotron lines from model accretion mounds on Neutron Stars

TL;DR

This study models how the distortion of magnetic fields caused by accreted matter on neutron stars affects the phase-dependent cyclotron spectra emitted from their hotspots, providing insights into the magnetic and accretion structure.

Contribution

It introduces a numerical solution of the Grad-Shafranov equation for accretion mounds and models the resulting cyclotron spectra, highlighting phase-dependent effects.

Findings

Magnetic field distortion influences cyclotron line features.

Spectral variations depend on viewing geometry.

Model provides a basis for interpreting observed spectra.

Abstract

In this paper we make a phase dependent study of the effect of the distortion of local magnetic field due to confinement of 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 MHD equilibria of 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.