THE EXTREMES OF CONTINUUM AND EMISSION-LINE VARIABILITY OF AGN: CHANGING-LOOK EVENTS AND BINARY SMBHS

The extremes of emission-line and continuum variability of Active Galactic Nuclei (AGN) are unique probes of the physics and geometry of the central engine. This review provides an overview of the most extreme cases of continuum and optical emission-line variability of AGN and the proposed interpretations. We also point out remaining challenges in the identification of changing-look (CL) AGN and discuss future prospects. This includes the need for identification of larger samples of CL narrow-line Seyfert 1 galaxies and CL LINERs at opposite ends of the accretion regime. In the second part, evidence for semi-periodic variability of broad lines and continuum emission, and its possible interpretation by the presence of binary SMBHs is addressed. Most recent results from the project MOMO are presented which monitors densely the best-known binary SMBH candidate OJ 287. In the last few years, the results from this project have ruled out the leading binary model and clearly established the need for new binary modeling in an entirely different parameter regime and based on the actually measured (primary) SMBH mass of 108 M (R). Ongoing and near-future time-domain surveys, and first gravitational wave detections of single systems, will play an important role in advancing this frontier in astrophysics.