Expanding the Landscape of Tidal Disruption Events

Massive black holes (black hole masses MBH ≳ 100 M☉) are key to our understanding of galaxy evolution, reionization, and physics that is as fundamental as general relativity. Many open questions remain surrounding MBH formation, growth, and demographics. We require methods of discovering BHs, as well as probes of accretion, that expand beyond analyses of the most luminous, accreting MBHs. Tidal disruption events (TDEs) occur when a star enters an orbit around an MBH with pericentric distance small enough that tidal forces from the MBH overcome the stellar self-gravity. They produce accretion flares on human timescales, lighting up otherwise quiescent MBHs and enabling direct observations of a newly formed accretion disk. Efforts to identify TDEs in optical, X-ray, and infrared survey data have led to the first populations of such events, but selection effects may be limiting our view of the TDE landscape; for example, we may be missing the lowest mass black holes or populations with certain types of host galaxies. In my thesis, I expand our understanding of the TDE landscape using novel TDE discovery methods. The first part of my thesis discusses eight examples of radio-selected TDEs with a range of multiwavelength properties. The first event was a long-lived, jetted TDE candidates, with evidence for ongoing energy injection into the jet despite a highly sub-Eddington accretion state, as has been observed in X-ray binary systems. The second event was an infrared, and radio bright TDE, which, despite showing no optical flare, can be modeled as a TDE analogous to optically-selected event but with significantly more dust and gas in the circumnuclear medium. I identified the final six events as radio-selected, optically-detected TDEs. I show that these events are largely analogous to the optically-selected events, despite being identified in a search with largely different selection effects. In the second part of my thesis, I expand to optical searches for nuclear transients, and I present two events that were identified in optical TDE searches aimed at identifying events that are excluded by typical selection criteria. The first is a candidate repeating, partial TDE, where a star is grazing the tidal radius on successive orbits and a small fraction of its mass is accreted. This event shows a fast-cooling, fast-evolving optical flare, unlike previously identified TDEs. Finally, I present a candidate accretion event onto an IMBH, identified in a search for hostless TDEs in optical data. My work shows the significant potential of current and upcoming surveys to identify flaring and variable MBHs in new regimes.