The thermal evolution of transiently accreting neutron stars

Abstract

Neutron stars are extremely dense objects that harbour extreme physical environments. They consist of a core with a radius of ~10 kilometers and a one-kilometer thick crust. Neutron stars in X-ray binaries accrete matter from a companion star. In transient systems accretion is not continuous, instead accretion outbursts are observed. During an outburst, material accumulates on the surface of the neutron star. This compresses the deeper layers of the crust and induces processes that release heat. Consequently, the neutron star crust heats up and becomes hotter than the core. After the outburst, the crust cools down to restore thermal equilibrium with the core. In this thesis, the thermal evolution of transiently accreting neutron stars is studied. We model the thermal evolution using the code NSCool and compare the results with observations to unravel properties of neutron stars. We improve our modelling methods by taking into account accretion rate variability (Chapter 2). We model the neutron star in KS 1731-260 and find that the constrained properties are strongly affected by variations in accretion rate. In Chapter 3 we model the 20-year outburst history of Aql X-1 and find that the neutron star does not have enough time between its frequent outbursts to restore crust-core equilibrium. Modelling Terzan 5 X-2 reveals that the neutron star in this system might have unusual crust properties (Chapter 4). In Chapter 5, we model how a neutron star reaches an equilibrium state over many outburst cycles and investigate how this equilibrium is affected by various parameters