Gravitational radiation from thermal mountains on accreting neutron stars: sources of temperature non-axisymmetry

TL;DR

This paper investigates how internal magnetic fields and thermal conductivity anisotropies in accreting neutron stars can create temperature asymmetries, leading to gravitational wave emission, with improved models suggesting detectable signals.

Contribution

It extends previous models by including the entire star and realistic microphysics, showing larger temperature asymmetries and potential for gravitational wave detection.

Findings

Temperature asymmetries can be up to 1000 times larger than previous estimates.

Magnetic fields of about 10^{12} G can generate significant asymmetries.

Enhanced models improve prospects for detecting gravitational waves from neutron stars.

Abstract

The spin-distribution of accreting neutron stars in low-mass X-ray binary (LMXB) systems shows a concentration of pulsars well below the Keplarian break-up limit. It has been suggested that their spin frequencies may be limited by the emission of gravitational waves, due to the presence of large-scale asymmetries in the internal temperature profile of the star. These temperature asymmetries have been demonstrated to lead to a non-axisymmetric mass-distribution, or `mountain', that generates gravitational waves at twice the spin frequency. The presence of a toroidal magnetic field in the interior of accreting neutron stars has been shown to introduce such anisotropies in the star's thermal conductivity, by restricting the flow of heat orthogonal to the magnetic field and establishing a non-axisymmetric temperature distribution within the star. We revisit this mechanism, extending the computational domain from (only) the crust to the entire star, incorporating more realistic microphysics, and exploring different choices of outer boundary condition. By allowing a magnetic field to permeate the core of the neutron star, we find that the likely level of temperature asymmetry in the inner crust ($\rho \sim 10^{13}$ g cm$^{-3}$) can be up to 3 orders of magnitude greater than the previous estimate, improving prospects for one day detecting continuous gravitational radiation. We also show that temperature asymmetries sufficiently large to be interesting for gravitational wave emission can be generated in strongly accreting neutron stars if crustal magnetic fields can reach $\sim 10^{12}$ G.