Gaussian Process Modeling Coronal X-ray Variability of Active Galactic Nuclei

TL;DR

This study models the X-ray variability of 13 active galactic nuclei using Gaussian Processes, revealing coronal timescales and suggesting internal or disk-driven mechanisms behind the variability.

Contribution

It applies Gaussian Process modeling to AGN X-ray light curves, providing new insights into coronal timescales and variability mechanisms.

Findings

Coronal X-ray timescales range from 3 to 50 days.

Some AGNs show timescales similar to optical disk timescales.

Variability may be driven by internal coronal processes or disk-corona interactions.

Abstract

The corona is an integral component of active galactic nuclei (AGNs) which can produce the X-ray emission. However, many of its physical properties and the mechanisms powering this emission remain a mystery. In this work, we study the coronal X-ray variabilities of 13 AGNs by Gaussian Process. 2-10 keV light curves of 13 AGNs can be successfully described by the damped-random walk (DRW) model. The extracted coronal X-ray timescales range from 3 to 50 days. In the plot of variability timescale versus black hole mass, the coronal X-ray timescales of four sources occupy almost the same region as the optical timescales of the accretion disk, with the latter matching the predicted thermal instability timescale of the disk. In contrast, the X-ray timescales of the remaining sources exhibit a systematic offset toward lower values. We propose that the coronal X-ray variability may be driven by internal processes within the corona itself (such as thermal conduction). On the other hand, it may also be triggered by local thermal instabilities occurring in different regions (close to the central black hole) of the accretion disk, which propagate to the corona via disk-corona coupling.