Relaxation of magnetically-confined mountains on accreting neutron stars through cross-field mass transport

TL;DR

This paper models how hydromagnetic instabilities and cross-field mass transport affect the structure and stability of magnetically confined mountains on accreting neutron stars, showing that mass transport can prevent mountain destruction.

Contribution

It introduces a self-consistent numerical scheme to study the impact of instability-driven cross-field mass transport on neutron star mountains, extending previous ideal MHD models.

Findings

Cross-field mass transport prevents mountain destruction by instabilities.

Mass-flux distribution adjusts to maintain a nonzero quadrupole moment.

The model compares unstable and artificially stabilized mountain configurations.

Abstract

Hydromagnetic instabilities modify the structure of a magnetically confined mountain on an accreting neutron star, once the accreted mass exceeds a critical value. Ideal magnetohydrodynamics and flux freezing break down, and mass diffuses across magnetic field lines locally, wherever instabilities are excited. Here a self-consistent, iterative, numerical scheme is used to evolve an axisymmetric magnetic mountain through a quasistatic sequence of Grad-Shafranov equilibria as a function of the accreted mass, $M_{\rm a}$, modified by instability-driven cross-field mass transport obeying the semi-analytic, Kulsrud-Sunyaev recipe. The results are compared to an artificially stabilised mountain, in which flux freezing does not break down, and there is no cross-field mass transport. It is shown that cross-field mass transport prevents instabilities from demolishing the mountain. Instead, the mass-flux distribution adjusts locally to nullify the instabilities and preserve a nonzero mass quadrupole moment indefinitely in the absence of ohmic dissipation.