Partially accreted crusts of neutron stars

Abstract

Neutron stars in low-mass binary systems are subject to accretion. The common assumption in studying the properties of the neutron star crust is the fully accreted crust approximation. However, observations of some X-ray transient sources indicate that the original crust has not been completely replaced by accreted material, but is partly composed of the compressed original crust. A two-part (or hybrid) crust made of the original crust that is compressed and of the accreted material crashing onto it was reconstructed as a function of the accretion stage. The differences in the composition and energy sources for the fully accreted and hybrid crusts influence the cooling and transport properties. A simple semi-empirical formula of a compressible liquid drop was used. We compared the nuclear reactions triggered by compression in the original crust and in the accreted matter part of the hybrid crust. We discuss another crust compression astrophysical phenomenon related to spinning neutron stars. The compression of the originally catalyzed outer crust triggers exothermic reactions (electron captures and pycnonuclear fusions) that deposit heat in the crust. The heat sources are cataloged as a function of the compression until the fully accreted crust approximation is reached. The pressure at which neutron drip occurs is a nonmonotonic function of the depth, leading to a temporary neutron drip anomaly. The additional potential source of energy for partially accreted crusts is the occurrence of a density inversion phenomenon between some compressed layers. The original crust of a neutron star cannot be neglected when the original crust is not fully replaced by the accreted matter. The amount of heat associated with the compression of the original crust is on the same order of magnitude as that from the sources acting in the accreted part of the hybrid crust.

17 pages, 10 figures, 4 files in Ancillary (2 movies, 2 .res files)

References in corpus (11)

• Models of crustal heating in accreting neutron stars
• Nucleosynthesis in Type I X-ray Bursts
• Cooling of Accretion-Heated Neutron Stars
• Supernova Fallback as Origin of Neutron Star Spins and Spin-kick Alignment
• Neutron drip transition in accreting and nonaccreting neutron star crusts
• Neutron reactions in accreting neutron stars: a new pathway to efficient crust heating
• Thermal states of neutron stars with a consistent model of interior
• Crust structure and thermal evolution of neutron stars in soft X-ray transients
• New insights on the puzzling LMXB 1RXS J180408.9-342058: the intermediate state, the clocked type-I X-ray bursts and much more
• Heating in Magnetar Crusts from Electron Captures
• IGR J17480-2446: a new class of accreting binaries?

Cited by in corpus (7)

• Heat transport and convective velocities in compositionally-driven convection in neutron star and white dwarf interiors
• Thermal evolution of neutron stars in soft X-ray transients with thermodynamically consistent models of the accreted crust
• Crystallization in single and multicomponent Neutron Star crusts
• Accreting neutron stars from the nuclear energy-density functional theory. II. Equation of state and global properties
• Virialized equation of state for warm and dense stellar plasmas in proto-neutron stars and Supernova matter
• Layers of electron captures in the crust of accreting neutron stars
• A "Hyperburst" in the MAXI J0556-332 Neutron Star: Evidence for a New Type of Thermonuclear Explosion