The Missing Pieces of the SMBH Accretion Puzzle: Revealing Extreme Accretion Modes

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How do supermassive black holes (SMBHs) turn their accretion “on” and “off”? How fast can they grow? How is this related to their circumnuclear environments? What are the consequences for the emergence of the first SMBHs and their co-evolution with galaxies? The key to answering these and other questions is found in understanding SMBH accretion, in all possible modes and cosmic epochs. Recent progress in both theory and observations strongly support “extreme” modes of SMBH accretion, namely abrupt changes seen on timescales of weeks and the possibility of (also long-lived) super-Eddington accretion. Both of these sorts of extreme accretion are not yet well understood. Here I propose to change this, by leading a multi-faceted observational program that will reveal, survey, and characterize extreme modes of SMBH accretion. Some of the efforts I will lead include: (1) Responsive, multi-wavelength, and spectroscopic follow-up observations of hyper-variable and flaring accreting SMBHs, to provide new insights as to what starts or stops SMBH accretion, and a new way to study super-Eddington accretion; (2) Proprietary multi-epoch SDSS-V spectroscopy will allow me to determine how common these dramatic events are, and to look for trends with BH and host galaxy properties; (3) A complete, detailed survey of highly accreting SMBHs in the local universe; and (4) An exploratory survey of super-Eddington, advection-dominated SMBHs at significant redshifts. These and other new insights will be combined with newly established, highly complete distributions of the black hole masses and accretion rates at both low and high redshifts, to understand the role of extreme accretion modes in the general SMBH population and to help guide future surveys. This research has to be done now, as we try to complete our understanding of (cosmic) SMBH accretion and (co-)evolution; before we are flooded with millions transients; and before the next generation facilities and surveys are finalized.