Speaker
Description
Accretion onto supermassive black holes powers the most luminous persistent sources in the Universe: active galactic nuclei (AGN). Their emission is characterized by two distinct spectral components: thermal optical/ultraviolet radiation from an optically thick accretion disk and a power-law X-ray tail from a corona located in the innermost regions. Despite their prominence, how radiatively efficient disks form and couple to the hot corona remains poorly understood.
In this talk, I will discuss six years of simultaneous ultraviolet and X-ray monitoring of the nearby active galaxy ESO511-G030. We witness an increase of the ultraviolet flux from the disk by more than an order of magnitude over a timescale of less than three years, which we interpret as the real-time formation of an optically thick accretion disk.
I will show that at accretion rates higher than approximately 1% of the Eddington limit, the ultraviolet and X-ray data are tightly coupled, following the well-defined non-linear correlation found in more luminous quasars. However, below this threshold, this relation breaks down abruptly. I will argue that this suggests the evaporation of the inner accretion disk into a geometrically thick, optically thin hot flow. This represents a spectacular example of an accretion-state transition analogous to those observed in stellar-mass black holes, confirming the need for a paradigm change in the models of radiatively efficient accretion flows around supermassive black holes.