Crustal lattice pressure as a source of neutron star mountains

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.

💡 Research Summary

The paper revisits the long‑standing hypothesis that temperature asymmetries in the crust of accreting neutron stars can generate a steady quadrupolar mass deformation (“mountain”) capable of emitting continuous gravitational waves (CGWs) strong enough to balance the accretion torque in low‑mass X‑ray binaries (LMXBs). Earlier work (Bildsten 1998; Ushomirsky, Cutler & Bildsten 2000, hereafter UCB) focused on the temperature‑dependent shift of electron‑capture layers (“capture layers”) in the crust. The idea is that a modest temperature perturbation (∼1 % of the crustal temperature) moves the depth at which a particular nuclear species captures electrons, thereby changing the local density and producing a quadrupole moment. By summing the contributions of many capture layers, UCB argued that the total quadrupole could reach the torque‑balance value \