New Types of Flares from Accreting Supermassive Black Holes

Our understanding of the evolving population of supermassive black holes (SMBHs) beyond the local universe is fundamentally limited to actively growing SMBHs, where relatively stable accretion of gas persists over several hundreds of millions of years.
A growing number of transient phenomena in galaxy nuclei have recently begun to shed new light on SMBH demographics and the physics of gas accretion onto these objects, tracing events where this accretion has drastically intensified, diminished, and/or otherwise disturbed. I will review some of these new classes of high-variability phenomena, focusing on new results obtained with responsive, multi-wavelength follow-up observations. These include “changing look AGN”, and other, yet poorly understood UV-bright flares from accreting SMBHs. While these events observationally differ from the (stellar) tidal disruption events known to date, the physics behind them may be interlinked. Together, these extreme events can greatly advance our understanding of SMBH accretion, teach us how and why SMBHs turn their accretion “on” and “off”, and reveal super-Eddington accretion. I will finally mention how new surveys, such as the SDSS-V, are going to discover many more transients from SMBHs.