Stars, black holes and explosive transients

Gamma-ray bursts and tidal disruption events are some of the most cataclysmic and energetic events in the universe. Both result in highly luminous and complex emission across the entire electromagnetic spectrum and such transients provide invaluable clues to the behaviour of matter under extreme forces and how the properties of the universe have arisen.

Short gamma-ray bursts are powered by the merger of a neutron star with either another neutron star or a stellar mass black hole. The extreme gravitational forces involved can squeeze neutron rich ejecta out of the neutron star and cause an additional fast evolving thermal optical transient called a kilonova. Kilonovae are sites of r-process nucleosynthesis and are responsible for the production of some of the heaviest elements in the universe. I have performed an in depth investigation of the short gamma-ray burst GRB 071227 and found it to have an optical excess. However, I found this excess to be inconsistent with a kilonova origin and instead is most likely due to the dust content of the source distorting the afterglow. A small excess does still remain, however, which could indicate a further thermal contribution.

Tidal disruption events, on the other hand, occur when a star approaches a supermassive black hole at an oblique angle. The enormous gravity of the black hole overcomes the selfgravity of the star and it is torn apart. The debris falls onto the black hole at high rates, possibly beyond what can be accreted by the black hole. How the black hole responds to such high fallback rates is currently poorly understood and here I have investigated one model of its behaviour, the zero-Bernoulli accretion model. I have produced optical light curves of such super-Eddington tidal disruption events and found them to have properties resembling those of observed events, although there were also signi?cant discrepancies and I therefore proposed improvements to the model. I also used the model light curves to evaluate the Gravitational-wave Optical Transient Observer's suitability for observing such events and derive optimal observing strategies. I found that up to 150 tidal disruption events could be observed per year, although this number is caveated by several assumptions.

Finally, I have used the Second Swift-XRT Point Source catalogue to search for previously unknown rare X-ray transients. Such efforts have been successfully applied to other X-ray catalogues to ?find new tidal disruption events, for instance. From the 206,335 sources in the catalogue, I identi?ed flaring extragalactic sources and analysed their light curves and other properties to classify them. This produced 19 new possible transients and I have explored some of their properties. I evaluated my methodology and found the fi?nal sample to have a completeness of ~ 65% and proposed improvements that could both improve the completeness and enable the identi?cation of transients in near real time with the future live version of the catalogue.