Exploring Exotic Transients: Stellar Explosions, Tidal Disruptions, and Compact Object Mergers in the Golden Age of Time-Domain Astronomy

Abstract

The advent of wide-field time-domain surveys has dramatically increased the transient discovery rate and directly led to uncovering new phenomena, such as superluminous supernovae (SLSNe) and tidal disruption events (TDEs), that challenge our understanding of the ways massive stars explode and the phenomena associated with supermassive black holes. Most recently, the field of multi-messenger astronomy has opened a new window into the universe through transients associated with compact object binary mergers, promising great advances in our knowledge of compact objects and the origin of the heaviest elements in the universe. This thesis explores these exotic transients using a two-pronged approach to time-domain astronomy that focuses on both transient characterization and environmental contextualization. First, my thesis addresses the challenge of identifying and characterizing SLSNe and TDEs by utilizing targeted selection methods to boost their identification rates in ongoing surveys. To date, I have achieved a SLSN/TDE identification rate of about 20%, a significant increase over previous efforts, and my extensive follow-up campaigns of identified SLSNe and TDEs have led to new insights. My program uncovered the first robust TDE candidate in a Seyfert galaxy, allowing the study of the interaction of the stellar debris with the pre-existing accretion disk. The emerging sample of TDEs in galaxies hosting an AGN suggests they may exhibit more efficient accretion than TDEs in quiescent galaxies. From deep late-time observations of hydrogen-poor SLSNe I have placed the most stringent constraints to date on the presence of radioactive material in fast evolving events. While this firmly rules out the decay of nickel as the dominant power source of the peak luminosities, at least some SLSNe have a large iron-group element abundance similar to supernovae associated with long gamma-ray bursts (LGRBs), providing a new link between SLSNe and engine-powered LGRBs. In addition, a very slowly evolving SLSN in my sample exhibits a late-time light curve signature suggestive of magnetar energy input. Second, I utilized galaxy environment studies to probe the progenitors of LGRBs and the progenitor system of the first binary neutron star merger, GW170817. Using a decade's worth of archival Hubble Space Telescope images of LGRB host galaxies, I measured the precise locations of these events within their hosts yielding insights about their progenitors. Finally, from studying the properties of GW170817's host galaxy, I placed constraints on the merger timescale, natal kicks, and the presence of an underlying star cluster.