The relationship between the X-ray variability and the central black hole mass

TL;DR

This study finds a significant inverse relationship between X-ray variability and black hole mass in active galactic nuclei, suggesting black hole size influences short-term X-ray fluctuations and explains variability differences across AGN types.

Contribution

It establishes a linear anti-correlation between X-ray variability and black hole mass, highlighting the black hole mass as a key parameter in AGN variability behavior.

Findings

X-ray variability is anti-correlated with black hole mass.

A linear relation sigma^2_{rms} ∝ M_{bh}^{-1} is suggested.

High-redshift QSOs show greater variability than local AGNs.

Abstract

We assembled a sample of Seyfert 1 galaxies, QSOs and Low-Luminosity Active Galactic Nuclei (LLAGNs) observed by ASCA, whose central black hole masses have been measured. We found that the X-ray variability (which is quantified by the ``excess variance'' sigma^2_{rms}) is significantly anti-correlated with the central black hole mass, and there likely exists a linear relationship sigma^2_{rms}\propto M^{-1}_{bh}. This can be interpreted that the short time-scale X-ray variability is caused by some global coherent variations in the X-ray emission region which is scaled by the size of the central black hole. Hence, the central black hole mass is the driving parameter of the previously established relation between X-ray variability and luminosity. This findings favor the hypothesis that the Narrow Line Seyfert 1 galaxies and QSOs harbor smaller black holes than the broad line objects, and can also easily explain the observational fact that high redshift QSOs have greater variability than local AGNs at a given luminosity. Further investigations are needed to confirm our findings, and a large sample X-ray variability investigation can give constraints on the physical mechanism and evolution of AGNs.