Accreting neutron stars from the nuclear energy-density functional theory. II. Equation of state and global properties

TL;DR

This study uses nuclear energy-density functional theory to analyze how accretion affects the equation of state and global properties of neutron stars, revealing increased stiffness and larger radii in accreting stars compared to non-accreting ones.

Contribution

It provides a unified, thermodynamically consistent calculation of the equation of state for accreting neutron stars using specific nuclear energy-density functionals, highlighting the role of nuclear shell effects.

Findings

Accreting neutron stars have a significantly stiffer equation of state.

Accreting neutron stars exhibit larger radii than catalyzed neutron stars.

Crustal moment of inertia and tidal deformability are minimally affected when density discontinuities are considered.

Abstract

The accretion of matter onto the surface of a neutron star in a low-mass X-ray binary triggers X-ray bursts, whose ashes are buried and further processed thus altering the composition and the properties of the stellar crust. In this second paper of a series, the impact of accretion on the equation of state and on the global properties of neutron stars is studied in the framework of the nuclear energy-density functional theory. Considering ashes made of $^{56}$Fe, we calculated the equations of state using the same Brussels-Montreal nuclear energy-density functionals BSk19, BSk20, and BSk21, as those already employed for determining the crustal heating in our previous study for the same ashes. All regions of accreting neutron stars were treated in a unified and thermodynamically consistent way. With these equations of state, we determined the mass, radius, moment of inertia, and tidal deformability of accreted neutron stars and compared with catalyzed neutron stars for which unified equations of state based on the same functionals are available. The equation of state of accreted neutron stars is found to be significantly stiffer than that of catalyzed matter, with an adiabatic index $\Gamma \approx 4/3$ throughout the crust. For this reason, accreting neutron stars have larger radii. However, their crustal moment of inertia and their tidal deformability are hardly changed provided density discontinuities at the interface between adjacent crustal layers are properly taken into account. The enhancement of the stiffness of the equation of state of accreting neutron stars is mainly a consequence of nuclear shell effects, thus confirming the importance of a quantum treatment as stressed in our first study. With our previous calculations of crustal heating using the same functionals, we have thus obtained consistent microscopic inputs for simulations of accreting neutron stars.