Heat release in accreting neutron stars

TL;DR

This paper derives a universal formula for heat release efficiency in accreting neutron stars' crusts, showing it depends mainly on the pressure at the crust interface, and finds previous estimates may be overestimated.

Contribution

It introduces a simplified, universal parametrization of crustal heating efficiency based on the pressure at the crust interface, refining previous models.

Findings

Heat release is primarily determined by the pressure at the outer-inner crust interface.

The heat release is significantly lower than previously estimated, by a factor of a few.

The model accounts for shell effects in nuclear models to estimate possible interface pressures.

Abstract

Observed thermal emission from accreting neutron stars (NSs) in a quiescent state is believed to be powered by nonequilibrium nuclear reactions that heat the stellar crust (deep crustal heating paradigm). We derive a simple universal formula for the heating efficiency, assuming that an NS has a fully accreted crust. We further show that, within the recently proposed thermodynamically consistent approach to the accreted crust, the heat release can be parametrized by the only one parameter -- the pressure $P_{\rm oi}$ at the outer-inner crust interface (as we argue, this pressure should not necessarily coincide with the neutron-drip pressure). We discuss possible values of $P_{\rm oi}$ for a selection of nuclear models that account for shell effects, and determine the net heat release and its distribution in the crust as a function of $P_{\rm oi}$. We conclude that the heat release should be reduced by a factor of few in comparison to previous works.