Consistency between X-ray and UV-Optical reverberation measurements in NGC 5548

Abstract

The hard X-ray$-$emitting hot corona is a key component of active galaxies. Constraints on the hot corona height can be derived from reverberation studies in both the X-ray and optical bands. X-ray reverberation (X-ray$-$RM) studies often imply a very low corona height, whereas UV/optical reverberation mapping (photometrcic continuum$-$RM) typically points to a much larger one. To reconcile this discrepancy, we examine the constraints provided by both methods for the same source. We adopt a uniform methodology using the {\tt KYNSED} and {\tt KYNXiltr} codes within a consistent modeling framework for reverberation mapping, applicable across both the X-ray and UV-optical spectral and time domains. We select the source NGC 5548, for which the necessary observational data are available in the literature. We carry out our analysis for NGC 5548, a source with extensive reverberation mapping data obtained independently in the X-ray and UV-optical bands across different epochs. Our results hint for a substantial discrepancy between the global parameters required to reproduce the X-ray and those needed to fit the UV-optical reverberation signals. In particular, the mismatch in the inferred black hole mass and accretion rate presents a significant challenge for interpreting the observed time delays within a unified reflection-based framework. Our unified reflection-based modeling sheds light on X-ray and UV-optical variability of NGC 5548, but discrepancies in black hole mass, accretion rate, and corona properties might imply fundamental challenges to a self-consistent model. However, future analyses leveraging extended X-ray dataset with improved treatment of absorption and variability coherence are crucial to obtaining more robust constraints.