Extreme Variability Reveals How the Eddington Ratio Regulates Coronal Power in Active Galactic Nuclei

Abstract

The bolometric luminosity ($L_{\rm bol}$) of active galactic nuclei (AGNs) is a key tracer of accretion physics, but its direct determination is often hindered by limited spectral coverage and contamination of the host galaxy. Bolometric corrections ($\kappa_{\lambda} = L_{\rm bol}/L_{\lambda}$) offer a practical means of estimating $L_{\rm bol}$, with the X-ray bolometric correction ($\kappa_{\rm 2-10}$) being crucial for exploring the coupling between the accretion disk and the X-ray corona. Here we present multi-epoch, multi-wavelength observations of five highly variable, changing-state AGNs that span more than three orders of magnitude in Eddington ratio ($-3.6\lesssim \log \lambda_{\rm Edd} \lesssim -0.5$). This unique data set reveals a remarkably tight relation between $\kappa_{\rm 2-10}$ and $\lambda_{\rm Edd}$, with an intrinsic scatter of only $\sim0.05$ dex. We find that while the sources show bolometric corrections following different tracks in luminosity space that depend on black hole mass, they all display the same $\kappa_{\rm 2-10}-\lambda_{\rm Edd}$ trend. This shows unambiguously that $\lambda_{\rm Edd}$ is the primary driver of X-ray bolometric corrections, and points to a tight underlying trend that can be used to obtain reliable estimates of bolometric output from X-ray luminosities. Our results highlight how time-domain, multi-wavelength observations of variable AGN offer unique insights into the accretion flow structure and its radiative output.