The universal shape of the X-ray variability power spectrum of AGN up to $z\sim 3$

TL;DR

This study demonstrates that the X-ray variability power spectrum of AGN follows a universal shape across a wide range of redshifts, luminosities, and black hole masses, with the bend frequency linked to black hole mass and possibly accretion rate.

Contribution

It introduces the variance-frequency diagram as a new method to analyze AGN variability and confirms the universality of the PSD shape up to redshift 3, revealing dependencies on black hole mass.

Findings

The PSD shape is consistent across different redshifts and luminosities.

The bend frequency correlates with black hole mass.

No significant evolution of PSD shape with redshift or luminosity.

Abstract

We study the ensemble X-ray variability properties of Active Galactic Nuclei (AGN) over a large range of timescales (20 ks $\leq T\leq$ 14 yrs), redshift ($0\leq z \lesssim 3$), luminosities ($10^{40}$ erg s$^{-1}\leq L_X\leq 10^{46}$ erg s$^{-1}$) and black hole (BH) masses ($10^6 \leq $M$_\odot \leq 10^9$). We propose the use of the variance-frequency diagram, as a viable alternative to the study of the power spectral density (PSD), which is not yet accessible for distant, faint and/or sparsely sampled AGN. We show that the data collected from archival observations and previous literature studies are fully consistent with a universal PSD form which does not show any evidence for systematic evolution of shape or amplitude with redshift or luminosity, even if there may be differences between individual AGN at a given redshift or luminosity. We find new evidence that the PSD bend frequency depends on BH mass and, possibly, on accretion rate. We finally discuss the implications for current and future AGN population and cosmological studies.