Nuclear Reactions in the Crusts of Accreting Neutron Stars

TL;DR

This paper details the nuclear reaction sequences in accreting neutron star crusts, revealing how composition and impurity levels evolve, which impacts thermal properties and observational interpretations.

Contribution

It provides a comprehensive analysis of nuclear reactions in neutron star crusts using a full network, highlighting differences from prior models and implications for crust impurity and thermal processes.

Findings

Inner crust impurity reduces regardless of initial composition

Shell effects can delay the formation of a pure crust

Nuclear heating is robust, but cooling depends on initial composition

Abstract

X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion, and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density is 2E12 g/cm^3 using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond 2E12 g/cm^3. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A >= 102 nuclei, where the inner crust remains impure with an impurity parameter of Qimp~20 due to the N = 82 shell closure. In agreement with previous work we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. This work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.