Nuclear Reactions in the Crusts of Accreting Neutron Stars

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 ρ = 2 × 10 12 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 ρ = 2 × 10 12 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 Q imp ≈ 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.

Crucial role of neutron diffusion in the crust of accreting neutron stars

“…Thus, the electron chemical potential should be a bit below beta-capture threshold, being almost constant until the total burnout of these nuclei. This statement agrees with already cited results by Lau et al 2018: the µe increases only of 0.1 MeV on course of density increase from ρ = 1.7 × 10 12 g cm −3 to ρ = 2.4 × 10 12 g cm −3 , where pycnonuclear reactions occur. As far as, Pei is mostly determined by electrons, it cannot increase strongly.…”

“…In this section, we argue that the absence of diffusion equilibrium is not a specific feature of our simplified reaction network Shchechilin & Chugunov (2019b,a), but it is a general feature of the traditional approach. First of all, in Shchechilin & Chugunov (2019a) we demonstrate that the composition of inner crust obtained within detailed reaction network by Lau et al (2018) for initial 56 Fe composition is well reproduced by our approach, thus the µe profile, corresponding to results of that work, based on FRDM95+DZ31 model, should be close to respective profile in figure 1, being not in diffusion equilibrium. In particular, Lau et al (2018) reports increase of µe only of 0.1 MeV on course of density increase from ρ = 1.7×10 12 g cm −3 to ρ = 2.4×10 12 g cm −3 , which should correspond to the region of weak dependence of µe on P in their calculations.…”

“…Consequently, the net result of the pycnonuclear reaction is the conversion of one atomic nucleus into neutrons (e.g. Lau et al 2018). The neutron number density is increased in the pycnonuclear region until all nuclei, participating in these reactions, are converted to neutrons (or increased neutron chemical potential prevent the disintegration of daughter nucleus to initial ones by neutron emissions).…”

“…40 Mg+ 40 Mg) or being triggered by beta capture (e.g. betacapture by 40 Mg triggers set of electon captures and neutron emissions leading to 25 N+ 40 Mg reaction in Lau et al 2018). Let us start with the second case.…”

Equation of state and composition of the inner crust of an accreting neutron star: multicomponent model

“…First calculations were made by Sato [6], the most widely used model was suggested by Haensel & Zdunik [7,8]. In the recent work Lau et al [9] constructed the reaction network model for the neutron star crust up to densities 2 × 10 12 g cm −3 . Here we present the model applicable up to ρ 2 × 10 13 g cm −3 .…”