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

Abstract

Correct interpretation of X-ray observations of transiently accreting neutron stars requires modeling of nuclear-physical processes in these objects. We consider a chain of nuclear reactions that drives the crust composition in an accreting neutron star and heats up the star. We constructed multicomponent approach with the kinetics of nuclear reactions described in simplified stepwise manner. The redistribution of nucleons between nuclei by emission and capture of neutrons is shown to significantly affect the nuclear reaction chains and the composition of the inner crust. In particular, even if the outer crust has one-component composition, the appearance of free neutrons in the inner crust leads to branching of reaction chains and formation of the multicomponent composition. We apply the compressible liquid drop nuclear model, which includes effects of free neutrons on nuclear energies. It allows us to calculate the composition, the heating profile and the equation of state of matter up to densities ρ 2 × 10 13 g cm −3 .

Heat release in accreting neutron stars

“…For the region of P oi depicted in the figure, Q ∼ (0.2 − 1) MeV/nucleon, being almost a linear function of P oi . It is by a factor of few smaller than the heat release ∼ (1.5 − 2) MeV/nucleon, found in the traditional approach [16,20,21,32]. Note that the shell…”

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

“…Only layers thicker than 1 m for 3 For MB model the accreted crust was recalculated according to priority rules formulated by Shchechilin & Chugunov (2019a). In Shchechilin & Chugunov (2019b) we apply a bit different priority rules, which leads to minor differences in composition of the accreted crust, but do not affect the features considered here. fiducial g = 2 × 10 14 cm s −2 are shaded.…”