Directly tracking the rebrightening of a supermassive black hole accretion disk

Accretion onto supermassive black holes (SMBHs) powers the most luminous persistent sources in the Universe, the so-called active galactic nuclei, whose emission is characterized by two distinct spectral components: thermal optical/ultraviolet (UV) radiation from an optically thick accretion disk and a power-law X-ray tail from a corona located in the innermost regions of the accretion flow. Yet, how radiatively efficient accretion disks develop and couple to the hot corona remains poorly understood. Using 6 yr of simultaneous UV and X-ray monitoring of the nearby active galaxy ESO 511-G030, we witness a dramatic evolution of the broadband spectral energy distribution, driven by an increase of the UV flux from the disk by more than 1 order of magnitude over a timescale of less than 3 yr. The overall behavior is unlikely to track an uncovering event and is instead compatible with a progressive recovery of the optically thick component of the accretion flow. At accretion rates higher than approximately 1% of the Eddington limit, UV and X-ray data are tightly coupled and follow the well-defined, nonlinear correlation between disk and corona found in the more luminous quasars. Below this threshold, the relation apparently breaks down, as expected in case of evaporation of the inner accretion disk into a geometrically thick, optically thin hot flow. This is a strong hint of an accretion-state transition analogous to those observed in stellar-mass black holes and confirms the need for a paradigm change in the models of radiatively efficient accretion flows around SMBHs.