Investigating AGN variability and Tidal Disruption Events with Wide Field Optical Surveys.

Active Galactic Nuclei (AGN) and Tidal Disruption Events (TDEs) are two phenomena that can lead to a deeper understanding of the supermassive black holes (SMBHs) that reside at the centres of all massive galaxies. Past optical surveys, such as the SDSS and PTF, have opened the way to studying both these phenomena using large samples of data. Current (e.g. GOTO, ZTF) and future (e.g LSST) surveys will provide long timespan, high cadence lightcurves for millions more galaxies than these earlier surveys, allowing us to study AGN variability and TDEs to an even greater level of detail. With this thesis, we aim to unveil the prospects of studying AGN variability and TDEs with GOTO - a recently-commissioned wide-field telescope based on La Palma, Spain. We use the data from the telescope's prototype phase to explore the variability of moderate luminosity and low redshfit ($z<0.4$) AGN. Most notably, we identify the barriers (e.g. systematic uncertainties) that currently prevent us from using this facility to study AGN variability for this sample, and suggest ways that the data could be improved to achieve this goal. Following the realisation that we are currently unable to study the variability of moderate luminosity AGN with GOTO, we turn to ZTF data to investigate the variability of our sample of AGN and how it relates to other physical parameters (i.e., luminosity, redshift, Eddington ratio, black hole mass), where we identify a slight anti-correlation between the amplitude of variability Eddington ratio. Using the prototype phase data, we have also investigated the potential of GOTO as a TDE discovery machine. Using a sample of known TDEs, which were identified by ZTF but are also found in GOTO archival data, we propose a number of ways, including host galaxy colour and pre-existed nuclear activity, to filter the GOTO data stream in order to recover these transient events. We also examine whether the difference imaging lightcurves generated from GOTO observations could be used to distinguish between TDEs and supernovae - a vital step if we wish to prioritise rapid follow-up observations. Over the coming years, GOTO's field-of-view is set to increase by a factor of four, with a corresponding increase in cadence. With this in mind, we also explore how these upgrades -- together with the longer lightcurves -- will improve the prospects of studying AGN variability and TDEs with GOTO. For AGN variability they allow better statistics, whereas for TDEs, they increase the number of detections, which in turn allows faster and more confident identification of candidates.