A falling Corona model for the anomalous behavior of the broad emission lines in NGC 5548

TL;DR

This paper proposes a model where a falling corona in NGC 5548 causes spectral and correlation anomalies in broad emission lines and UV/X-ray emissions, explaining observed variability and lag changes.

Contribution

It introduces a simple falling corona model that accounts for the anomalous broad emission line behavior and spectral variability in NGC 5548, supported by observational correlations.

Findings

The correlation between broad emission lines and UV continua weakens during anomalies.

The time lag between X-ray and UV emissions increases and varies with the state.

The model explains dependence on velocity and ionization energy in the observed anomalies.

Abstract

NGC 5548 has been intensively monitored by the AGN Space Telescope and Optical Reverberation Mapping collaboration. Approximately after half of the light curves, the correlation between the broad emission lines and the lag-corrected ultraviolet continua becomes weak. This anomalous behavior is accompanied by an increase of soft X-ray emission. We propose a simple model to understand this anomalous behavior, i.e., the corona might fall down, thereby increasing the covering fraction of the inner disk. Therefore, X-ray and extreme ultraviolet emission suffer from spectral variations. The ultraviolet continua variations are driven by both X-ray and extreme ultraviolet variations. Consequently, the spectral variability induced by the falling corona would dilute the correlation between the broad emission lines and the ultraviolet continua. Our model can explain many additional observational facts, including the dependence of the anomalous behavior on velocity and ionization energy. We also show that the time lag and correlation between the X-ray and the ultraviolet variations change as NGC 5548 displays the anomalous behavior. The time lag is dramatically longer than the expectation from disk reprocessing if the anomalous behavior is properly excluded. During the anomalous state, the time lag approaches the light-travel timescale of disk reprocessing albeit with a much weaker correlation. We speculate that the time lag in the normal state is caused by reprocessing of the broad line region gas. As NGC 5548 enters the abnormal state, the contribution of the broad line region gas is smaller; the time lag reflects disk reprocessing. We also discuss alternative scenarios.