On the Black Hole Mass---X-ray Excess Variance Scaling Relation for Active Galactic Nuclei in the Low-mass Regime

TL;DR

This study investigates whether the inverse linear scaling between X-ray variability and black hole mass in AGN extends to low-mass black holes below 10^6 solar masses, finding that the relation flattens at lower masses.

Contribution

It provides the first detailed analysis of X-ray excess variance in low-mass AGN, revealing a deviation from the linear scaling at masses below 10^6 solar masses.

Findings

The scaling relation flattens at around 10^6 solar masses.

The flattening aligns with the shape of the AGN power spectrum density.

The results support the inverse relation between break frequency and black hole mass.

Abstract

Recent studies of active galactic nuclei (AGN) found a statistical inverse linear scaling between the X-ray normalized excess variance $\sigma_{\rm rms}^2$ (variability amplitude) and the black hole mass spanning over $M_{\rm BH}=10^6- 10^9\ M_{\odot}$. Being suggested to have a small scatter, this scaling relation may provide a novel method to estimate the black hole mass of AGN. However, a question arises as to whether this relation can be extended to the low-mass regime below $\sim10^6\ M_{\odot}$. If confirmed, it would provide an efficient tool to search for AGN with low-mass black holes using X-ray variability. This paper presents a study of the X-ray excess variances for a sample of AGN with black hole masses in the range of $10^5- 10^6\ M_{\odot}$ observed with {\it XMM-Newton} and {\it ROSAT}, including data both from the archives and from newly preformed observations. It is found that the relation is no longer a simple extrapolation of the linear scaling; instead, the relation starts to flatten at $\sim10^6\ M_{\odot}$ toward lower masses. Our result is consistent with the recent finding of \citet{L15}. Such a flattening of the $M_{\rm BH}-\sigma_{\rm rms}^2$ relation is actually expected from the shape of the power spectrum density of AGN, whose break frequency is inversely scaled with the mass of black holes.