Heat release in accreting neutron stars

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 only one parameter—the pressure $P_{\mathrm{oi}}$ 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 $P_{\mathrm{oi}}$ for a selection of nuclear models that account for shell effects, and we determine the net heat release and its distribution in the crust as a function of $P_{\mathrm{oi}}$. We conclude that the heat release should be reduced by a factor of few in comparison to previous works.

Figure 1

The heat release $Q$, $Q_{\mathrm{o}}$, $Q_{\mathrm{oi}}$, and $Q_{\mathrm{i}}$ vs $P_{\mathrm{oi}}$ for several nuclear models. BSk24, BSk25, BSk26: Hartree-Fock-Bogoliubov calculations [31]; FRDM12: finite-range droplet macroscopic model [32]. Arrows indicate the neutron drip pressure for catalyzed crust, $P_{\mathrm{nd}}^{(\mathrm{cat})}$. For each model, the pressure region above the neutron drip point for AC (calculated within the traditional approach) is shaded gray.