The X-ray/UV connection in NGC 5548: A rapidly varying corona

¹ Department of Physics, University of Crete 71003 Heraklion, Greece
² Institute of Astrophysics, FORTH GR-71110 Heraklion, Greece
³ Department of Physics and Institute of Theoretical and Computational Physics, University of Crete 71003 Heraklion, Greece
⁴ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology Cambridge MA 02139, USA
⁵ Cahill Center for Astronomy and Astrophysics, California Institute of Technology 1200 California Boulevard Pasadena CA 91125, USA
⁶ Astronomical Institute of the Academy of Sciences Boční II 1401 CZ-14100 Prague, Czech Republic

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Received: 10 October 2025
Accepted: 4 December 2025

Abstract

Recent intensive monitoring campaigns of active galactic nuclei have provided simultaneous X-ray, UV, and optical data of unprecedented quality. The observations revealed a strong correlation between the UV and optical variability, but a weaker correlation between the X-ray and UV bands. This challenges the standard X-ray reprocessing scenario. We revisit the X-ray/UV connection in NGC 5548 by fitting archival 2014 HST and Swift/XRT light curves assuming X-ray reverberation from a dynamically evolving X-ray corona. Our results show that as long as the corona height, photon index, and power vary over time, X-ray reverberation can explain the observed UV and optical variability within 2% and 5%, respectively (on average). The evolution of the best-fit parameters suggests that fast changes in coronal geometry and energetics on a timescale of days are required to explain the observed variability.