Nuclear physics of the outer layers of accreting neutron stars

Abstract

Now 50 years since the existence of the neutron star crust was proposed, we review the current understanding of the nuclear physics of the outer layers of accreting neutron stars. Nuclei produced during nuclear burning replace the nascent composition of the neutron star ocean and crust. Non-equilibrium nuclear reactions driven by compression alter the outer thermal structure and chemical composition, leaving observable imprints on astronomical phenomena. As observations of bursting neutron stars and cooling neutron stars have increased, the recent volume of astronomical data allows new insights into the microphysics of the neutron star interior and the possibility to test nuclear physics input in model calculations. Despite numerous advances in our understanding of neutron star interiors and observed neutron star phenomena, many challenges remain in the astrophysics theory of accreting neutron stars, the nuclear theory of neutron-rich nuclei, and the reach and precision of terrestrial nuclear physics experiments. Nuclear Physics of the Outer Layers of Accreting Neutron StarsFor example, current investigations of the critical reaction rates in X-ray bursts [9,27], studies of key properties of nuclei in the accreted crust [25,26], and nuclear reaction network calculations of accreted crust compositions [22,24,28], all promise to improve the observational constraints on dense matter derived from accreting neutron stars.We begin in section 2 by briefly outlining the neutron star structure. Sections 3 and 4 describe the original composition of the neutron star crust and the accretion process which drives the system from equilibrium. In section 5, we discuss the nuclear burning that can occur on the surface of accreting neutron stars and the nuclei produced during the different possible burning regimes. In actively accreting neutron stars, the ashes of prior surface nuclear burning are compressed to greater depths by newly accreted material. We discuss in section 6 the nuclear interactions involving the ashes that take place as the ambient mass density increases. In section 7, we investigate the impact of interior nuclear interactions on observable neutron star phenomena. We summarize and discuss prospects for future work in section 8.

Treating quarks within neutron stars

“…Observations of the surface temperatures of accreting neutron stars in their quiescent periods have indeed confirmed the presence of a crust (see Ref. [2], and references therein). This region is characterized by a Coulomb lattice of neutron-rich nuclei surrounded by dripped neutrons with admixtures of light nuclei and a uniform background of electrons in chemical potential and pressure equilibrium in a charge-neutral state.…”

Thermal evolution and quiescent emission of transiently accreting neutron stars

“…Although the latter radius is more realistic, we have rescaled the corresponding values of Ṁ obs in Table 2 (lines 17, 18, 21, 22, 25, 32, and 34) back to the canonical model for the uniformity of the data sample. The total (bolometric) accretion luminosity measured at infinity is related to the accretion rateṀ measured locally at the neutron star surface by equation (e.g., Mitra 1998;Meisel et al 2018)…”

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

“…As a final remark, let us point that construction of diffusion equilibrium crust, in principle, can be important for the shallow heating problem -the phenomenological powerful heating source of unknown nature, which is introduced to the models of crustal cooling to explain observational data (Meisel et al 2018;Degenaar et al 2019;Parikh et al 2019). This source is typically assumed to be localized in outer crust, thus it is rather unlikely that neutron diffusion provides a direct mechanism to explain this source.…”