Sinking of a magnetically confined mountain on an accreting neutron star

TL;DR

This study uses magnetohydrodynamic simulations to analyze how magnetic mountains on neutron stars affect their shape and gravitational wave emission, revealing that sinking into a fluid base significantly reduces ellipticity.

Contribution

It provides the first detailed simulations comparing the effects of a hard surface versus a sinking fluid base on magnetic mountain ellipticity in neutron stars.

Findings

Ellipticity reaches ~2e-4 for accreted masses > 1.2e-3 solar masses.

Sinking reduces ellipticity by 25% to 60%.

Implications for gravitational wave signals from low-mass X-ray binaries.

Abstract

We perform ideal-magnetohydrodynamic axisymmetric simulations of magnetically confined mountains on an accreting neutron star, with masses less than ~0.12 solar masses. We consider two scenarios, in which the mountain sits atop a hard surface or sinks into a soft, fluid base. We find that the ellipticity of the star, due to a mountain grown on a hard surface, approaches ~2e-4 for accreted masses greater than ~1.2e-3 solar masses, and that sinking reduces the ellipticity by between 25% and 60%. The consequences for gravitational radiation from low-mass x-ray binaries are discussed.