The effect of diffusive nuclear burning in neutron star envelopes on cooling in accreting systems

Valuable information about the neutron star (NS) interior can be obtained by comparing observations of thermal radiation from a cooling NS crust with theoretical models. Nuclear burning of lighter elements that diffuse to deeper layers of the envelope can alter the relation between surface and interior temperatures and can change the chemical composition over time. We calculate new temperature relations and consider two effects of diffusive nuclear burning (DNB) for H–C envelopes. First, we consider the effect of a changing envelope composition and find that hydrogen is consumed on short time-scales and our temperature evolution simulations correspond to those of a hydrogen-poor envelope within ∼100 d. The transition from a hydrogen-rich to a hydrogen-poor envelope is potentially observable in accreting NS systems as an additional initial decline in surface temperature at early times after the outburst. Second, we find that DNB can produce a non-negligible heat flux, such that the total luminosity can be dominated by DNB in the envelope rather than heat from the deep interior. However, without continual accretion, heating by DNB in H–C envelopes is only relevant for <1–80 d after the end of an accretion outburst, as the amount of light elements is rapidly depleted. Comparison to crust cooling data shows that DNB does not remove the need for an additional shallow heating source. We conclude that solving the time-dependent equations of the burning region in the envelope self-consistently in thermal evolution models instead of using static temperature relations would be valuable in future cooling studies

The thermal evolution of transiently accreting neutron stars

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