Explaining the moderate UV/X-ray correlation in AGN

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.

7 pages, 4 figures, accepted for publication by ApJ

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• Continuum optical-UV and X-ray variability of AGN: current results and future challenges
• Broadband X-ray/UV/optical time-resolved spectroscopy of NGC 5548: The origin of the UV/optical variability in active galactic nuclei
• Contrasting X-ray/UV time-lags in Seyfert 1 galaxies NGC 4593 and NGC 7469 using AstroSat observations
• Connecting the X-ray/UV variability of Fairall 9 with NICER: A Possible Warm Corona
• Applying wavelet analysis to the X-ray light curves of active galactic nuclei and quasi-periodic eruptions
• Detection of X-ray/UV delay in NGC 4051 using AstroSat observations
• Searching for Quasi-Periodic Eruptions using Machine Learning
• Testing X-ray Reprocessing and Mapping the Soft Excess of NGC 7469 with NICER
• The UV/optical Continuum Reverberation Mapping of Eight Active Galactic Nuclei with Swift: Further Evidence for the Outer Component
• Untangling the Complex Nature of AGN Variability with Fairall 9
• Intensive X-ray/UVOIR continuum reverberation mapping of the Seyfert AGN MCG+08-11-11
• The X-ray/UV Connection in NGC 5548: A Rapidly Varying Corona