Explaining the moderate UV/X-ray correlation in AGN

TL;DR

This paper demonstrates that the moderate UV/X-ray correlation observed in AGN can be explained by the variability of the X-ray source's geometry and physical configuration, aligning observations with theoretical models.

Contribution

It shows that geometric and physical variability of the X-ray source can produce the observed moderate correlations, reconciling observations with the X-ray illumination paradigm.

Findings

Variability of the X-ray source affects UV/X-ray correlation.

Geometric configuration changes produce observed correlation ranges.

Low UV/X-ray correlation does not contradict X-ray driven UV variability.

Abstract

The UV/optical and X-ray variability of active galactic nuclei (AGN) have long been expected to be well correlated as a result of the X-ray illumination of the accretion disk. Recent monitoring campaigns of nearby AGN, however, found that their X-ray and UV/optical emission are only moderately correlated, challenging the aforementioned paradigm. In this work, we aim to demonstrate that due to the definition of the cross correlation function, a low UV/X-ray correlation is well expected in the case of an X-ray illuminated accretion disk, when the dynamic variability of the X-ray source is taken into account. In particular, we examine how the variability of the geometric or physical configuration of the X-ray source affects the expected correlation. Variations of the geometric configuration are found to produce a range of UV/X-ray cross correlations, which match well the observed values, while they result in high correlation between the UV and optical variability, reconciling the observed results with theoretical predictions. We conclude that the detection of a low UV/X-ray correlation does not contradict the assumption of the UV/optical variability being driven by the X-ray illumination of the disk, and we discuss the implications of our results for correlation studies.