Exploring black-hole scaling relations via the ensemble variability of Active Galactic Nuclei

TL;DR

This paper introduces an empirical model linking AGN demographics to their X-ray variability, providing new insights into black-hole scaling relations and accretion physics by comparing model predictions with observational data.

Contribution

The study presents the first model connecting AGN population demographics with their ensemble X-ray variability, constraining the PSD shape and black-hole mass relations.

Findings

Disfavors a broken power-law PSD with constant normalization.

Indicates PSD amplitude depends on accretion properties like Eddington ratio.

Supports local spheroid black-hole mass relations based on current data.

Abstract

An empirical model is presented that links, for the first time, the demographics of AGN to their ensemble X-ray variability properties. Observations on the incidence of AGN in galaxies are combined with (i) models of the Power Spectrum Density (PSD) of the flux variations of AGN and (ii) parameterisations of the black-hole mass vs stellar-mass scaling relation, to predict the mean excess variance of active black-hole populations in cosmological volumes. We show that the comparison of the model with observational measurements of the ensemble excess variance as a function of X-ray luminosity provides a handle on both the PSD models and the black-hole mass vs stellar mass relation. We find strong evidence against a PSD model that is described by a broken power-law and a constant overall normalisation. Instead our analysis indicates that the amplitude of the PSD depends on the physical properties of the accretion events, such as the Eddington ratio and/or the black hole mass. We also find that current observational measurements of the ensemble excess variance are consistent with the black-hole mass vs stellar mass relation of local spheroids based on dynamically determined black-hole masses. We also discuss future prospects of the proposed approach to jointly constrain the PSD of AGN and the black-hole mass vs stellar mass relation as a function of redshift.