Understanding the Most Powerful Explosions in the Universe

Gamma-ray bursts (GRBs) are the most luminous transient events in the Universe. The population of observed GRBs is organised into two categories: long and short, separated by a two second divide in gamma-ray emission duration. The short type (lasting less than two seconds) have been shown to originate from the merger of two neutron stars, whereas as long bursts (lasting longer than two seconds) originate from the collapse of massive stars. There are subtypes within both classes that challenge the standard model for GRBs. For shorts, some bursts exhibit a re-brightening in their high-energy emission becoming dominant shortly after the initial emission spike known as extended emission bursts. For long bursts, some exhibit flares in their X-ray afterglows that contain a comparable amount of energy to the prompt emission. These are so-called giant X-ray flares. This thesis examines the central engine that drives these extreme types of bursts since they have the potential to discern between various proposed GRB models. A potential explanation for these events may be a highly magnetised, rapidly rotating neutron star (magnetar) fed by fallback accretion. The motivation for using this model is the late-time plateaux seen in some short GRBs that can be interpreted as a long-lived magnetar losing angular momentum along magnetic field lines. The fallback accretion component extends the global energy budget of the system and allows the rotational energy reservoir of the magnetar to be refreshed.