Update on the X-ray variability plane for active galactic nuclei: The role of the obscuration

TL;DR

This study refines the X-ray variability plane for active galactic nuclei by demonstrating the influence of obscuration on the PSD break frequency, using XMM-Newton data to establish a new relation involving black hole mass and line-of-sight absorption.

Contribution

It introduces a revised variability plane incorporating obscuration as a key parameter, supported by analysis of short-term PSD variations in a sample of 22 AGN.

Findings

PSD break frequency varies in most AGN analyzed.

Obscuration (NH) significantly affects the variability relation.

The new relation aligns with previous models when obscuration is minimal.

Abstract

Scaling relations are the most powerful astrophysical tools to set constraints to the physical mechanisms of astro- nomical sources and to infer properties that cannot be accessed directly. We re-investigate here one of these scaling relations in active galactic nuclei (AGN); the so-called X-ray variability plane (or mass-luminosity-timescale relation, McHardy et al. 2006). This relation links the power-spectral density (PSD) break frequency with the super-massive black hole (SMBH) mass and the bolometric luminosity. We used available XMM -Newton observations of a sample of 22 AGN to study the PSD and spectra in short segments within each observation. This allows us to report for the first time that the PSD break frequency varies for each object, showing variations in 19 out of the 22 AGN analyzed. Our analysis of the variability plane confirms the relation between the break frequency and the SMBH mass and finds that the obscuration along the line of sight NH (or the variations on the obscuration using its standard deviation, $\rm{\Delta}$(NH)) is also a required parameter, at least for the range of frequencies analyzed here (3x10E-5-5x10E-2Hz). We constrain a new variability plane of the form: log($\nu_{Break}$) = (-0.589$\rm{\pm}$0.005) log(MBH ) + (0.10$\rm{\pm}$0.01) log(NH ) - (1.5$\rm{\pm}$0.3) (or log($\nu_{Break}$)=(-0.549$\rm{\pm}$0.009)log(MBH)+(0.56$\rm{\pm}$0.06)$\rm{\Delta}$(NH)+(0.19$\rm{\pm}$0.08)). The X-ray variability plane found by McHardy et al. (2006) is roughly recovered when we use unobscured segments. We speculate that this behavior is well explained if most of the reported frequencies are related to inner clouds (within 1pc), following Kepler orbits under the gravitational field of the SMBH.