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

Abstract

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 ρ ≤ 2 × 1012 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, Poi, which encodes all uncertainties related to the physics of the deepest inner-crust layers. Our calculations allow us to set new theoretical lower limits on the net energy release (per accreted baryon): Q ≳ 0.28 MeV for extreme rp ashes and Q ≳ 0.43–0.51 MeV for superburst and Kepler ashes. Our results can be directly incorporated into numerical codes and provide an opportunity to constrain Poi by comparing thermal evolution models of accreting neutron stars with observations.