The X-ray/UV Connection in NGC 5548: A Rapidly Varying Corona

TL;DR

This study investigates the X-ray and UV variability in NGC 5548, showing that a dynamic X-ray corona with rapidly changing properties can explain observed UV and optical fluctuations, challenging standard models.

Contribution

It demonstrates that time-evolving coronal geometry and energetics are necessary to account for the observed X-ray/UV variability in NGC 5548.

Findings

X-ray reverberation explains UV/optical variability within a few percent.

Coronal parameters vary significantly over days.

Rapid coronal changes are essential to match observations.

Abstract

Recent intensive monitoring campaigns of active galactic nuclei (AGN) have provided simultaneous X-ray, UV, and optical data of unprecedented quality. The observations reveal a strong correlation between the UV and optical variability, but a weaker correlation between the X-ray and UV bands, challenging 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 time scale of days are required to explain the observed variability.