Journal article Open Access

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

Antonis Georgakakis; Maurizio Paolillo; Iossif Papadakis

An empirical model is presented that links 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.

The mock AGN catalogue used in the paper is made available. The contents are described in the README file. The code used in the paper is available at Full details can be found in the associated paper at

The code that uses this data is available at The paper describing the modeling approach is available at
Files (1.5 GB)
Name Size
4.4 kB Download
1.5 GB Download

All versions This version
Views 108108
Downloads 88
Data volume 4.5 GB4.5 GB
Unique views 101101
Unique downloads 66


Cite as