Directly tracking the re-brightening of a supermassive black hole accretion disk

TL;DR

This study observes the evolution of a supermassive black hole's accretion disk and corona over six years, revealing state transitions and coupling behaviors similar to stellar-mass black holes, challenging existing accretion models.

Contribution

First direct evidence of accretion-state transition in a supermassive black hole, linking disk-corona coupling to accretion rate thresholds and suggesting paradigm shifts in accretion flow models.

Findings

Ultraviolet flux increased by over an order of magnitude in less than three years.

Tight coupling between UV and X-ray emission above 1% Eddington, with a non-linear correlation.

Breakdown of the UV-X-ray relation below 1% Eddington, indicating disk evaporation.

Abstract

Accretion onto supermassive black holes 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 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 six years of simultaneous ultraviolet 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 ultraviolet flux from the disk by more than an order of magnitude over a time scale of less than three years. 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 one per cent of the Eddington limit, ultraviolet and X-ray data are tightly coupled and follow the well-defined, non-linear 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 supermassive black holes.