# Anomalous behaviour of the UV-optical continuum bands in NGC 5548

## TL;DR

During a 2014 multi-wavelength campaign of NGC 5548, the UV-optical emission lines showed unusual behavior, revealing that the ionising continuum was weaker and softer during the anomaly, with implications for understanding AGN emission mechanisms.

## Contribution

This study provides the first evidence of anomalous behavior in longer wavelength continuum bands and infers the properties of the variable EUV continuum affecting the broad-line region.

## Key findings

- The ionising continuum was weaker and softer during the anomaly.
- Evidence of anomalous behavior in longer wavelength continuum bands.
- The variable UV-optical continuum partly originates from diffuse gas emission.

## Abstract

During the 2014 HST/Swift and ground-based multi-wavelength monitoring campaign of NGC 5548 (AGN STORM), the UV-optical broad emission lines exhibited anomalous, decorrelated behaviour relative to the far-UV continuum flux variability. Here, we use key diagnostic emission lines (Ly-alpha and He II) for this campaign to infer a proxy for the all important, variable driving EUV continuum incident upon BLR clouds. The inferred driving continuum provides a crucial step towards the recovery of the broad emission line response functions in this AGN. In particular, the ionising continuum seen by the BLR was weaker and softer during the anomalous period than during the first third of the campaign, and apparently less variable than exhibited by the far-UV continuum. We also report the first evidence for anomalous behaviour in the longer wavelength (relative to 1157A) continuum bands. This is corroborative evidence that a significant contribution to the variable UV-optical continuum emission arises from a diffuse continuum emanating from the same gas that emits the broad emission lines.