Supermassive Black Holes in X-Rays: From Standard Accretion to Extreme Transients

Abstract

X-rays are a critical wavelength for understanding supermassive black holes (SMBHs). X-rays probe the inner accretion flow, closest to the event horizon, where gas inspirals, releasing energy and driving black hole growth. This region also governs the launching of outflows and jets that regulate galaxy evolution and link SMBH growth to their host galaxies. This review focuses on X-ray observations of SMBHs, through “standard accretion” in persistent active galactic nuclei (AGN) and in extreme transient events, such as tidal disruption events (TDEs), changing-look AGN, and quasi-periodic eruptions (QPEs). We describe the X-ray spectral and variability properties of AGN and the observational techniques that probe the inner accretion flow. By understanding the phenomenology and accretion physics in standard, individual AGN, we can better probe more exotic phenomena, including binary SMBH mergers or extreme mass ratio inspirals (EMRIs). In this review, the reader will discover the following: ▪ X-ray variability on timescales from minutes to hours traces accretion near the event horizon. ▪ X-ray can measure the black hole mass, spin, and accretion flow geometry and dynamics. ▪ In transients like TDEs, X-rays probe the newly formed accretion disk that feeds the black hole. ▪ QPEs are posited to be EMRIs orbiting accreting SMBHs that would emit low-frequency gravitational waves. ▪ Future X-ray, time-domain, and multimessenger surveys will revolutionize our understanding of SMBH growth.