The Impact of Neutron Transfer Reactions on Heating and Cooling of Accreted Neutron Star Crusts

TL;DR

This study incorporates neutron transfer reactions into nuclear network models of accreted neutron star crusts, revealing their significant impact on crust composition, impurity, and thermal processes, while confirming the robustness of previous heating and cooling conclusions.

Contribution

It introduces the first comprehensive inclusion of neutron transfer reactions in crust models, identifying a new reaction cycle and assessing their effects on crust properties.

Findings

Neutron transfer reactions are prevalent and influence crust composition.

A new reaction cycle involving neutron transfer alters nuclear equilibrium.

Previous models' conclusions on heating and cooling remain largely valid.

Abstract

Nuclear reactions heat and cool the crust of accreting neutron stars and need to be understood to interpret observations of X-ray bursts and of long-term cooling in transiently accreting systems. It was recently suggested that previously neglected neutron transfer reactions may play a significant role in the nuclear processes. We present results from full nuclear network calculations that now include these reactions and determine their impact on crust composition, crust impurity, heating, and cooling. We find that a large number of neutron transfer reactions indeed occur and impact crust models. In particular, we identify a new type of reaction cycle that brings a pair of nuclei across the nuclear chart into equilibrium via alternating neutron capture and neutron release, interspersed with a neutron transfer. While neutron transfer reactions lead to changes in crust model predictions, and need to be considered in future studies, previous conclusions concerning heating, cooling, and compositional evolution are remarkably robust.