The Accretion Discs of Transient X-ray Binaries

One of the larger questions within the study of X-ray Binaries currently concerns the geometry of the accretion disc during the so called ’hard state’. A model involving a truncated disc is often used to explain the properties of the hard state, but there is still disagreement on the extent to which it is necessary. Presented in this thesis are three studies related to this issue. The first looks into the changes in the accretion geometry using an argument based around radiative efficiency. Periods of exponential decay before and after the soft- to hard-state transition are found. The e-folding times of these decays are measured and it is found that this value changes from ~12 days in the soft state to ~7 days in the hard state. This factor ~2 change would be expected if there is a change from a radiatively efficient regime to a radiatively inefficient regimee. The second concerns the treatment of absorption from the Interstellar Medium in studies of X-ray Binaries. Column densities for the most abundant elements are found and compared with previous results for a number of sources. Simulated data is also used to test the impact of using incorrect column densities and older X-ray absorption models on spectral analysis. It is found that the use of incorrect absorption parameters can have a large effect on the results of spectral fitting. The third directly tests for the presence of a truncated disc by fitting a model to energy spectra from the XMM-Newton/EPIC-pn instrument. The model assumes the accretion disc extends down to rISCO at all times, and is fit to a large range of observations in both the hard and soft states. In the majority of cases there is no explicit need for a truncated disc in either the soft or hard state.