Gravitational Waves from Neutron Stars with Large Toroidal B-fields

TL;DR

Neutron stars with large toroidal magnetic fields can become strong gravitational-wave sources due to magnetic distortions causing secular instabilities, which may be detectable by advanced gravitational-wave observatories.

Contribution

This paper demonstrates that large toroidal magnetic fields in neutron stars induce instabilities leading to significant gravitational-wave emission, a novel insight into neutron star evolution and gravitational-wave sources.

Findings

Toroidal B-fields distort neutron stars into prolate shapes.

Secular instability causes wobble angle growth, enhancing GW emission.

Potential detectability of GW signals from neutron stars with strong toroidal B-fields.

Abstract

We show that NS's with large toroidal B-fields tend naturally to evolve into potent gravitational-wave (gw) emitters. The toroidal field B_t tends to distort the NS into a prolate shape, and this magnetic distortion can easily dominate over the oblateness ``frozen into'' the NS crust. An elastic NS with frozen-in B-field of this magnitude is clearly secularly unstable: the wobble angle between the NS's angular momentum J^i and the star's magnetic axis n_B^i grow on a dissipation timescale until J^i and n_B^i are orthogonal. This final orientation is clearly the optimal one for gravitational-wave (gw) emission. The basic cause of the instability is quite general, so we conjecture that the same final state is reached for a realistic NS. Assuming this, we show that for LMXB's with B_t of order 10^{13}G, the spindown from gw's is sufficient to balance the accretion torque--supporting a suggestion by Bildsten. The spindown rates of most millisecond pulsars can also be attributed to gw emission sourced by toroidal B-fields, and both these sources could be observed by LIGO II. While the first-year spindown of a newborn NS is most likely dominated by em processes, reasonable values of B_t and the (external) dipolar field B_d can lead to detectable levels of gw emission, for a newborn NS in our own galaxy.