Changing-Look AGN Powered By Disk Tearing

Abstract

Changing-look active galactic nuclei (CLAGN) feature order-of-magnitude variability in both the continuum and broad line luminosities on months-to-years long timescales, and are currently unexplained. Simulations have demonstrated that rotating black holes sometimes tear apart tilted accretion disks. These tearing events violently restructure the disk on timescales much shorter than a viscous timescale, hinting at a connection to CLAGN. Here, we show that disk tearing can power changing-look events. We report synthetic observations of an extremely high resolution three-dimensional general-relativistic magnetohydrodynamic simulation of a geometrically thin, tilted accretion disk around a rapidly rotating, $10^8\,M_\odot$ black hole. We perform ray-tracing calculations that follow the disk light to both a line of sight camera and to a distribution of cameras in a prescribed torus-like broad line region. The continuum photoionizes the broad line region and we calculate the resulting spectrum. Both the continuum and line luminosities undergo order of magnitude swings on months-to-years long timescales. We find shorter, weeks long variability driven by the geometric precession of the inner disk and an intraday quasi-periodic oscillation driven by radial breathing of the inner disk. When the torn disk precesses, it causes asymmetric illumination of the broad line region, driving time-evolving red-to-blue asymmetries of the broad emission lines that may be a smoking gun for disk tearing. We also make predictions for future photometric observations from ULTRASAT and Vera Rubin Observatory, both of which may play an important role in detecting future changing-look events.