MHD instabilities in accretion mounds - 1: 2D axisymmetric simulations

TL;DR

This study uses 2D MHD simulations to analyze the stability of accretion mounds on neutron stars, revealing stability thresholds and differences in spectral features between filled and hollow mounds.

Contribution

It provides the first detailed stability analysis of accretion mounds using 2D MHD simulations, highlighting the role of excess mass and mound structure.

Findings

Mounds are stable against interchange instabilities.

Excess mass destabilizes the mounds beyond a threshold.

Hollow mounds produce wider, more complex CRSF profiles.

Abstract

We have performed stability analysis of axisymmetric accretion mounds on neutron stars in High Mass X-ray Binaries (HMXB) by 2-D MHD simulations with the PLUTO MHD code. We find that the mounds are stable with respect to interchange instabilities, but addition of excess mass destabilizes the equilibria. Our simulations confirm that accretion mounds are unstable with respect to MHD instabilities beyond a threshold mass. We investigate both filled and hollow mounds and the for the latter also compute the expected profile of cyclotron resonance scattering features (CRSF). In comparison to CRSF from filled mounds reported in our earlier work, hollow mounds display wider and more complex line profiles.