Probing the Central Engine of Active Galactic Nuclei with Variability

Active Galactic Nuclei (AGN) provide a unique laboratory to study black hole accretion and the physical structure of the central engine across cosmic time. Variability is a fundamental observable in this context, enabling direct measurements of the physical size scales of the emitting regions, including the accretion disk, broad-line region, and dusty environment. Time lags and multi-wavelength time-domain observations offer one of the few methods to probe these otherwise unresolved regions and to constrain the physics governing energy generation near supermassive black holes. Recent advances in high-cadence monitoring, reverberation mapping, and time-domain analysis techniques have transformed variability into a precision tool for studying AGN inner structure. The upcoming generation of time-domain facilities will further expand these capabilities, enabling variability-based studies across a wide range of redshifts and luminosities. This splinter session aims to bring together the German AGN community to discuss how time-domain observations across wavelengths and redshifts can be combined to build a coherent physical picture of AGN inner regions, including extreme variability phenomena and supermassive black hole transients. Particular emphasis will be placed on cross-wavelength connections, systematic uncertainties, and preparation for upcoming time-domain surveys such as LSST, combined with high-resolution follow-up from space- and ground-based facilities including JWST, ELT instrumentation such as MICADO, and interferometric capabilities like GRAVITY+.