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.
Highlights
Correlated continuum and broad emission line variability studies have proven a powerful probe of the central regions of Active Galactic Nuclei (AGN)
Assuming that the λ1157 ̊A continuum band faithfully tracks the continuum at ionising energies3, and that the emission-line variations arise from a narrow range in delays, we can infer a suitable proxy for the driving continuum as seen by the broad emission line region by shifting in time and scaling in amplitude, the observed emission line light curve to match the observed UV continuum variations outside of the anomalous period
The reduced variability and larger than expected delays measured at all wavelengths, and in particular in the vicinity of the Balmer continuum, relative to that predicted by the standard disk model (Edelson et al 2015, 2017, 2018; Fausnaugh et al 2016; Starkey et al 2017, McHardy et al 2018) we argue is broadly consistent with what one might expect if the UV–optical continuum bands are significantly contaminated by a diffuse continuum emitting from the same gas that emits the broad emission lines (KG01; Lawther et al 2018)
Summary
Correlated continuum and broad emission line variability studies (reverberation mapping, hereafter RM) have proven a powerful probe of the central regions of Active Galactic Nuclei (AGN). Following the seminal paper by Blandford & McKee (1982), it was soon realised that RM could reveal the spatial distribution and kinematics of the line emitting gas, and, with few assumptions, provide an estimate for the mass of the central super-massive black hole (e.g., Peterson et al 2002, 2014; Bentz et al 2009, 2010a,b, 2014; Grier et al 2012; Kaspi et al 2002, 2007; Pancoast et al 2012, 2014a,b). RM can be used to study the dominant components occupying the central regions of AGN on small (disk), intermediate (BLR), and large (dusty torus) scales, and the relationship(s) between them
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