Simulation of X-ray Bursts and Superbursts

Abstract

Observations of neutron star in binary systems provide powerful constrains on the physics at the surface of neutron stars. During the accretion of matter from the companion star, periodic nuclear explosion are triggered in the outer layers of the neutron star, increasing the luminosity during a time range of a few minutes. Rarely, one can also detect day-long explosions in accreting binary systems. The nature of those two kind of bursts is still not well understood. In fact, simplified simulations of the outer layers of an accreting neutron star in a binary are not yet able to reproduce all observable features. The work presented in this thesis is devoted to the one-dimensional simulations of X-ray bursts and superbursts. The numerical code used in this work has initially been programmed by J. Fisker in 2006. By updating and optimizing the code, we are able to simulate X-ray bursts as well as superbursts in a feasible time range. Using a large nulear network, we study the features of X-ray bursts and compare them with observations. To understand the link between various properties entering our simulations as parameters or boundary conditions, we present several models which reproduces hunderds of X-ray burst. In this current work, we focus mainly on changes in crustal heating, accretion rate and accretion composition. Analyzing the influence on the light curve as well as on the ashes of X-ray bursts, we are able to compare our results with observations. To shed some light on the self-consistent ignition of a superburst, we model a setup which may lead to the ignition of a superburst. Our results suggest that additional helium, heavier isotopes and the lack of hydrogen in the accretion composition help to generate carbon-rich X-ray burst ashes. Strong heating below the superburst ignition layer prevents the destruction of carbon after an X-ray bursts and might be the key ingredience in the self-consistent ignition of a superburst within the time range of the observed recurrence time