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-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-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-oi. We conclude that the heat release should be reduced by a factor of few in comparison to previous works.

Automated summary

Thermal emission from accreting neutron stars in a quiescent state is believed to be powered by nonequilibrium nuclear reactions heating the stellar crust. A universal formula for the heating efficiency is derived assuming a fully accreted crust, with the heat release parameterized by the pressure at the outer-inner crust interface. The heat release is expected to be reduced by a factor of few compared to previous works.