Accreting neutron stars: heating of the upper layers of the inner crust

TL;DR

This paper presents thermodynamically consistent calculations of heat release in the crust of accreting neutron stars, considering different nuclear ash models, to improve understanding of their thermal evolution.

Contribution

It introduces a new method to calculate energy release in neutron star crusts across different zones, accounting for uncertainties in deep crust physics.

Findings

Lower limits on energy release per accreted baryon: 0.28 MeV for Extreme rp ashes.

Energy release estimates: 0.43-0.51 MeV for Superburst and Kepler ashes.

Provides constraints for thermal evolution models of accreting neutron stars.

Abstract

Neutron stars in low-mass X-ray binaries are thought to be heated up by accretion-induced exothermic nuclear reactions in the crust. The energy release and the location of the heating sources are important ingredients of the thermal evolution models. Here we present thermodynamically consistent calculations of the energy release in three zones of the stellar crust: at the outer-inner crust interface, in the upper layers of the inner crust (up to the density $\rho \leq 2\times 10^{12}$ g cm$^{-3}$), and in the underlying crustal layers. We consider three representative models of thermonuclear ashes (Superburst, Extreme rp, and Kepler ashes). The energy release in each zone is parametrized by the pressure at the outer-inner crust interface, which encodes all uncertainties related to the physics of the deepest inner-crust layers. Our calculations allow us, in particular, to set new lower limits on the net energy release (per accreted baryon): $Q\gtrsim0.28$ MeV for Extreme rp ashes and Q~0.43-0.51 MeV for Superburst and Kepler ashes.