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

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.

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

“…Our physics input includes the FRDM12 model for nuclear masses and the BSK24 model to describe unbound neutrons. These models allow us to perform calculations up to the density ρ 2 × 10 12 g cm −3 , where the predictions of FRDM12 mass table can still be considered as reasonable (Lau et al 2018;Schatz et al 2021). The resulting heat release is shown in Fig.…”

“…In SGC21, we restrict our consideration to nuclear evolution in the outer crust and, using equation (1), calculate deep crustal heating for multicomponent ashes (three representative compositions were considered) within the nHD approach. In this Letter, we extend the calculation to the upper layers of the inner crust, limited by the density ρ dc = 2 × 10 12 g cm −3 , where it is still reasonable to use the atomic mass tables (Lau et al 2018;Schatz et al 2021). We explicitly calculate the heating at the oi interface, Qoi, and the heating in the upper layers (ρ < ρ dc ) of the inner crust, Qi1.…”

“…However, the 'traditional' models of accreting NSs (Sato 1979;Haensel & Zdunik 1990Gupta et al 2008;Steiner 2012;Fantina et al 2018;Lau et al 2018;Shchechilin & Chugunov 2019;Schatz et al 2021) have missed an important element. Namely, all of them implicitly neglect the redistribution of unbound neutrons in the inner crust.…”