BASS LIII: The Eddington Ratio as the Primary Regulator of the Fraction of X-ray Emission in Active Galactic Nuclei

TL;DR

This study demonstrates that the Eddington ratio primarily governs the X-ray emission fraction in AGN, revealing a shift in accretion physics at low Eddington ratios through analysis of 236 unobscured AGN.

Contribution

It identifies the Eddington ratio as the main driver of X-ray bolometric corrections, clarifying its role over luminosity in AGN emission models.

Findings

X-ray bolometric correction correlates with Eddington ratio.

Dependence on luminosity diminishes at low Eddington ratios.

Potential change in accretion physics at low Eddington ratios.

Abstract

Active galactic nuclei (AGN) emit radiation via accretion across the entire energy spectrum. While the standard disk and corona model can somewhat describe this emission, it fails to predict specific features such as the soft X-ray excess, the short-term optical/UV variability, and the observed UV/X-ray correlation in AGN. In this context, the fraction of AGN emission in different bands (i.e., bolometric corrections) can be useful to better understand the accretion physics of AGN. Past studies have shown that the X-ray bolometric corrections are strongly dependent on the physical properties of AGN, such as their luminosities and Eddington ratios. However, since these two parameters depend on each other, it has been unclear which is the main driver of the X-ray bolometric corrections. We present here results from a large study of hard X-ray-selected (14-195 keV) nearby ($z<0.1$) AGN. Based on our systematic analysis of the simultaneous optical-to-X-ray spectral energy distributions of 236 unobscured AGN, we found that the primary parameter controlling the X-ray bolometric corrections is the Eddington ratio. Our results show that while the X-ray bolometric correction increases with the bolometric luminosity for sources with intermediate Eddington ratios ($0.01-1$), this dependence vanishes for sources with lower Eddington ratios ($<0.01$). This could be used as evidence for a change in the accretion physics of AGN at low Eddington ratios.