Systematic Collapse of the Accretion Disc Across the Supermassive Black Hole Population

TL;DR

This study uses X-ray and optical data to show that supermassive black hole accretion discs transition from a standard, luminous state to a radiatively inefficient, hot plasma state at about 2% of the Eddington luminosity, revealing key accretion physics.

Contribution

It provides observational evidence for a universal accretion disc collapse transition in AGN across cosmic time, using uncontaminated spectra from the eROSITA eFEDS survey.

Findings

Disc-dominated AGN at high luminosity transition to hot plasma at ~0.02 L_Edd.

Transition observed across a wide AGN population, confirming theoretical predictions.

Spectral changes are not due to obscuration or host galaxy contamination.

Abstract

The structure of the accretion flow onto supermassive black holes (SMBH) is not well understood. Standard disc models match to zeroth order in predicting substantial energy dissipation within optically-thick material producing a characteristic strong blue/UV continuum. However they fail at reproducing more detailed comparisons to the observed spectral shapes along with their observed variability. Based on stellar mass black holes within our galaxy, accretion discs should undergo a transition into an X-ray hot, radiatively inefficient flow, below a (mass scaled) luminosity of $\sim 0.02\,L_{\rm{Edd}}$. While this has been seen in limited samples of nearby low-luminosity active galactic nuclei (AGN) and a few rare changing-look AGN, it is not at all clear whether this transition is present in the wider AGN population across cosmic time. A key issue is the difficulty in disentangling a change in spectral state from increased dust obscuration and/or host galaxy contamination, effectively drowning out the AGN emission. Here we use the new eROSITA eFEDS Survey to identify unobscured AGN from their X-ray emission, matched to excellent optical imaging from Subaru's Hyper Suprime-Cam; allowing the subtraction of the host galaxy contamination. The resulting, uncontaminated, AGN spectra reveal a smooth transition from a strongly disc dominated state in bright AGN, to the collapse of the disc into an inefficient X-ray plasma in the low luminosity AGN, with the transition occurring at $\sim 0.02\,L_{\rm{Edd}}$; revealing fundamental aspects of accretion physics in AGN.