Modelling mountains on accreting magnetized neutron stars

Abstract

Continuous gravitational waves from accreting neutron stars in Low Mass X-ray Binaries are one of the main targets for current and next generation ground based detectors. In order to select the most promising astrophysical sources, however, reliable predictions for the signals are required, and it is therefore necessary to develop models that consistently account for the combined effects of magnetic stresses, accretion-induced heating, and the elastic response of the crust.}{We present a model for computing the quadrupolar deformation, incorporating for the first time the coupled effects of a poloidal magnetic field, deep crustal heating, and crustal elasticity. Perturbations to the star's structure driven by the Lorentz force density and by thermally-induced density variations are computed by solving a system of linearised deformation equations in the crust, for which we consider the full elastic response, while the ocean and core treated as barotropic fluids. We identify a threshold accretion rate whose value depends on crustal microphysics and the superfluid gaps in the core, above which magnetic stresses and asymmetric accretion drive deformations of opposite sign, while below this threshold their roles are reversed. The predicted eccentricities reach magnitudes up to $\varepsilon\sim 10^{-11}$, corresponding to characteristic gravitational-wave strains accessible to next-generation detectors such as the Einstein Telescope or Cosmic Explorer, but generally below the sensitivity of current LIGO, Virgo and KAGRA interferometers. These results are consistent with the non-detection of continuous gravitational waves from accreting neutron stars in Low Mass X-ray Binaries in recent observational campaigns, but highlight the need of reliable models to understand the impact of gravitational wave emission in these systems and select relevant targets for future searches.