Three-dimensional stability of magnetically confined mountains on accreting neutron stars

TL;DR

This paper investigates the three-dimensional stability of magnetically confined mountains on accreting neutron stars using ideal-MHD simulations, revealing stability against certain instabilities and potential gravitational wave signals.

Contribution

It extends previous axisymmetric stability analysis to three dimensions, demonstrating the stabilizing effect of boundary conditions and characterizing the nonaxisymmetric saturation state.

Findings

Axisymmetric equilibria are stable against the Parker instability sub-mode.

Boundary conditions at the stellar surface are crucial for stability.

The predicted gravitational wave amplitude is within detection range of Advanced LIGO.

Abstract

We examine the hydromagnetic stability of magnetically confined mountains, which arise when material accumulates at the magnetic poles of an accreting neutron star. We extend a previous axisymmetric stability analysis by performing three-dimensional simulations using the ideal-magnetohydrodynamic (ideal-MHD) code \textsc{zeus-mp}, investigating the role played by boundary conditions, accreted mass, stellar curvature, and (briefly) toroidal magnetic field strength. We find that axisymmetric equilibria are susceptible to the undular sub-mode of the Parker instability but are not disrupted. The line-tying boundary condition at the stellar surface is crucial in stabilizing the mountain. The nonlinear three-dimensional saturation state of the instability is characterized by a small degree of nonaxisymmetry ($\la 0.1$ per cent) and a mass ellipticity of $\epsilon \sim 10^{-5}$ for an accreted mass of $M_a = 10^{-5} M_\odot$. Hence there is a good prospect of detecting gravitational waves from accreting millisecond pulsars with long-baseline interferometers such as Advanced LIGO. We also investigate the ideal-MHD spectrum of the system, finding that long-wavelength poloidal modes are suppressed in favour of toroidal modes in the nonaxisymmetric saturation state.