Exploring the Link between the X-ray Power Spectra and Energy Spectra of Active Galactic Nuclei

TL;DR

This study investigates how the X-ray power spectral density (PSD) of active galactic nuclei (AGN) varies with spectral states, revealing that PSD shape and high-frequency breaks depend on flux states and spectral variability.

Contribution

It provides the first detailed analysis of PSD evolution with spectral states in AGN, showing correlations between spectral changes and variability properties.

Findings

PSD shape varies with spectral state and flux level

High-frequency PSD breaks depend on the spectral state

X-ray rms variability can change by up to 1 dex between states

Abstract

Active Galactic Nuclei (AGN) are generally considered as the scaled-up counterparts of X-ray binaries (XRBs). It is known that the power spectral density (PSD) of the X-ray emission of XRBs shows significant evolution with spectral states. It is not clear whether AGN follow a similar evolutionary trend, however, though their X-ray emission and the PSD are both variable. In this work, we study a sample of nine AGN with multiple long observations with XMM-Newton, which exhibit significant X-ray spectral variation. We perform Bayesian PSD analysis to measure the PSD shape and variation. We find that a large change in the X-ray energy spectrum (mainly the change of flux state) is often accompanied by a large change in the PSD shape. The emergence of a high-frequency break in the PSD also depends on the spectral state. Among the four sources with significant high-frequency PSD breaks detected, three show the break only in the high-flux state, while the remaining one shows it only in the low-flux state. Moreover, the X-ray rms variability in different spectral states of an AGN is found to vary by as much as 1.0 dex. These results suggest that the different variability properties observed are likely caused by different physical processes dominating different spectral states. Our results also indicate that the intrinsic PSD variation can introduce a significant fraction of the dispersion as reported for the correlations between various X-ray variability properties and the black hole mass.