Magnetic field structure in accretion columns on HMXB and effects on CRSF

TL;DR

This paper models the magnetic field structure in accretion columns of neutron stars in HMXBs, showing how local field distortions influence cyclotron resonance scattering features and depend on mass loading and stability conditions.

Contribution

It introduces a numerical approach to model accretion mounds and analyzes how mass loading affects magnetic field distortion and CRSF shapes in HMXBs.

Findings

Local magnetic field distortion significantly alters CRSF profiles.

Mass loading variations change the shape of accretion mounds.

Stability of mounds depends on total accreted mass and mass loading patterns.

Abstract

In accreting neutron star binaries, matter is channelled by the magnetic fields from the accretion disc to the poles of neutron stars forming an accretion mound. We model such mounds by numerically solving the Grad-Shafranov equation for axisymmetric static MHD equilibria. From our solutions we infer local distortion of field lines due to the weight of accreted matter. Variation in mass loading at the accretion disc will alter the shape of the accretion mound which will also affect the local field distortion. From simulations of cyclotron resonance scattering features from HMXBs, we conclude that local field distortion will greatly affect the shape and nature of the CRSF. From phase resolved spectral analysis one can infer the local field structure and hence the nature of mass loading of field lines at the accretion disc. We also study the stability of such mounds by performing MHD simulations using the PLUTO MHD code. We find that pressure and gravity driven instabilities depend on the total mass accreted and the nature of mass loading of the field lines.