Crustal lattice pressure as a source of neutron star mountains

TL;DR

This paper proposes that temperature-induced changes in the crustal lattice pressure of neutron stars could generate mass asymmetries, or mountains, potentially explaining gravitational wave emission limits in low-mass X-ray binaries, without requiring deep capture layers.

Contribution

It introduces a novel mechanism where crustal lattice pressure perturbations, driven by temperature asymmetries, may produce neutron star mountains, challenging previous models dependent on deep capture layers.

Findings

Temperature-induced perturbations in crustal lattice pressure may be significant.

This mechanism does not require accretion, applicable to isolated neutron stars.

Further detailed calculations are needed to confirm viability.

Abstract

The spin frequencies of neutron stars in low-mass X-ray binaries may be limited by the emission of gravitational waves. A candidate for producing such steady emission is a mass asymmetry, or "mountain", sourced by temperature asymmetries in the star's crust. A number of studies have examined temperature-induced shifts in the crustal capture layers between one nuclear species and another to produce this asymmetry, with the presence of capture layers in the deep crust being needed to produce the required mass asymmetries. However, modern equation of state calculations cast doubt on the existence of such deep capture layers. Motivated by this, we investigated an alternative source of temperature dependence in the equation of state, coming from the pressure supplied by the solid crustal lattice itself. We show that temperature-induced perturbations in this pressure, while small, may be significant. We therefore advocate for more detailed calculations, self-consistently calculating both the temperature asymmetries, the perturbations in crustal lattice pressure, and the consequent mass asymmetries, to establish if this is a viable mechanism for explaining the observed distribution of low-mass X-ray binary spin frequencies. Furthermore, the crustal lattice pressure mechanism does not require accretion, extending the possibility for such thermoelastic mountains to include both accreting and isolated neutron stars.