The thermal and compositional structure of the crust of an accreting neutron star.

摘要

This thesis discusses the atmospheres, oceans, and crusts of accreting neutron stars. On accreting X-ray pulsars, the magnetic field focuses the accretion flow onto a small fraction of the stellar area and produces rapid local accretion rates in excess of the Eddington limit. The strong magnetic field then confines the accreted matter to depths where the lateral, pressure greatly exceeds B2/8π. The currents needed to confine the mountain are large enough to modify, by order unity, the magnetic field strength at the polar cap. Rapid compression at local accretion rates exceeding ten times the Eddington rate heats the atmosphere/ocean to temperatures of order 109 K at relatively low densities; for stars accreting pure helium, this causes unstable ignition of the ashes (mostly carbon) resulting from stable helium burning. This unstable carbon ignition can recur on timescales shorter than a day.; The ashes of hydrogen/helium burning eventually replace the original crust of a neutron star in a long-lived, low-mass x-ray binary. This replaced crust is of much lower thermal and electrical conductivity and contains layers of non-equilibrium reactions, which heat the deep crust and core. The thermal structure of the crust depends most sensitively on the composition of the crust at densities near neutron drip and is particularly insensitive (at near-Eddington accretion rates, appropriate for the brightest low-mass binaries) to the temperatures in the atmosphere. The deep crustal heating and low electrical conductivity drastically shorten the timescale for the diffusion of magnetic flux, and may be an important influence in the evolution of the magnetic field.; A signature of deep crustal heating is the quiescent emission from the slowly accreting neutron star transients. The heat released in the crust by the non-equilibrium reactions is thermally radiated from the neutron star surface during quiescence. The predicted quiescent luminosity has the correct magnitude to explain observations. Since the quiescent luminosity sets an upper bound to the core temperature, observations of neutron star transients also constrain the amount of heating by, e.g., viscous dissipation of a steady state r-mode.