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

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.