Taming the derivative: diagnostics of the continuum and H$\beta$ emission in a prototypical Population B active galaxy

Abstract

We report preliminary results on the analysis of the continuum and H$\beta$ light curves of the type-1 active galactic nucleus (AGN) NGC 5548. We notice a clear signature of shallowing in the trend between the H$\beta$ and the continuum luminosities. We attempt the recovery of this observed H$\beta$ emission trend as a response to large continuum flux increase using CLOUDY photoionization simulations. We explore a wide range in the physical parameters space for modelling the H$\beta$ emission from the broad-line region (BLR) appropriate for this source. We employ a constant density, single cloud model approach in this study and successfully recover the observed shallowing of the H$\beta$ emission with respect to the rising AGN continuum. With our modelling, we are able to provide constraints on the local BLR cloud density and recover the BLR distances (from the continuum source) consistent with the H$\beta$ reverberation mapping estimates. We further discuss the implications of the BLR covering factor and their sizes on recovering the observed trend.

10 pages, 7 figures, accepted version after proofs in Astronomische Nachrichten (Special Issue, 13th Serbian Conference on Spectral Line Shapes in Astrophysics)

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• Unveiling the quasar main sequence: illuminating the complexity of active galactic nuclei and their evolution
• Application of the FRADO model of BLR formation to the Seyfert galaxy NGC 5548 and the first step toward determining the Hubble constant
• HALO I: Photometric continuum reverberation mapping of Fairall 9
• Rarefied Broad-Line Regions in Active Galactic Nuclei: Anomalous Responses in Reverberation Mapping and Implications for Weak Emission-Line Quasars