Generation of strong magnetic fields by r-modes in millisecond accreting neutron stars: induced deformations and gravitational wave emission

TL;DR

This paper investigates how r-mode instabilities in accreting millisecond neutron stars can generate strong magnetic fields, induce deformations, and produce gravitational waves, potentially explaining observed spin limits.

Contribution

It explicitly models magnetic damping in r-mode evolution, showing magnetic field growth to $10^{14}$ G and resulting in gravitational wave emission from neutron star deformations.

Findings

Magnetic fields up to $10^{14}$ G can develop in neutron star cores.

Induced magnetic deformations lead to ellipticities around $10^{-8}$.

Gravitational wave emission may explain spin frequency limits.

Abstract

Differential rotation induced by the r-mode instability can generate very strong toroidal fields in the core of accreting, millisecond spinning neutron stars. We introduce explicitly the magnetic damping term in the evolution equations of the r-modes and solve them numerically in the Newtonian limit, to follow the development and growth of the internal magnetic field. We show that the strength of the latter can reach large values, $B \sim 10^{14} $ G, in the core of the fastest accreting neutron stars. This is strong enough to induce a significant quadrupole moment of the neutron star mass distribution, corresponding to an ellipticity $|\epsilon_B}| \sim 10^{-8}$. If the symmetry axis of the induced magnetic field is not aligned with the spin axis, the neutron star radiates gravitational waves. We suggest that this mechanism may explain the upper limit of the spin frequencies observed in accreting neutron stars in Low Mass X-Ray Binaries. We discuss the relevance of our results for the search of gravitational waves.