Magnetically confined mountains on accreting neutron stars with multipole magnetic fields

TL;DR

This paper models magnetically confined mountains on accreting neutron stars with multipole magnetic fields, showing that strong multipole fields significantly increase ellipticity and influence magnetic field burial and transformation.

Contribution

It introduces a numerical formulation for mountains with multipole magnetic fields and explores their effects on ellipticity and magnetic field evolution.

Findings

Mass ellipticity increases with strong multipole magnetic fields.

Buried multipole fields can sustain intense toroidal magnetic fields.

Magnetic field burial transforms dipole and multipole components.

Abstract

Magnetically confined mountains on accreting neutron stars are candidates for producing continuous gravitational waves. We formulate a magnetically confined mountain on a neutron star with strong multipole magnetic fields and obtain some sequences of numerical solutions. We find that the mass ellipticity of the mountain increases by one order of magnitude if the neutron star has strong multipole magnetic fields. As matter accretes on to the magnetic pole, the size of the mountain increases and the magnetic fields are buried. If the neutron star has a dipole magnetic field, the dipole magnetic field is buried and transformed into multipole components. By contrast, if the neutron star has both dipole and strong multipole magnetic fields, the multipole magnetic fields are buried and transformed into a negative dipole component. We also calculate magnetically confined mountains with toroidal magnetic fields and find that the ellipticity becomes slightly smaller when the mountain has toroidal magnetic fields. If the multipole magnetic fields are buried, they sustain the intense toroidal magnetic field near the stellar surface, and the ratio of the toroidal magnetic field to the poloidal magnetic field is close to 100. The hidden strong toroidal magnetic fields are sustained by the buried multipole magnetic fields.