Advances in studies of stellar tidal disruption events, AGN and quasi-periodic eruptions

In recent years, the advent of wide-field optical imaging surveys has opened our eyes to some of the most energetic events in the Universe. These include extreme brightness variations from active and passive supermassive black holes in the nearby Universe. While these objects are initially identified in the optical band, X-ray follow-up is proving to be essential for understanding the underlying physical mechanisms in these systems. With a large effective area in soft X-rays (0.25-10 keV), good spectral resolution, and superior ability to respond promptly to transient triggers and keep monitoring them, the NICER mission has made remarkable strides in our understanding of these sources. I will present recent scientific advances enabled by NICER in the fields of stellar tidal disruption events (TDEs) and active galactic nuclei (AGN). I will describe NICER discoveries of quasi-periodic eruptions from the nuclei of nearby galaxies (candidates for extreme mass-ratio inspirals), the formation (and destruction) of X-ray coronae around supermassive black holes in TDEs and changing-look AGN, and evidence of powerful winds/outflows in some of these systems. I will also describe new ways to measure spins of massive black holes using TDEs.