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

TL;DR

This study analyzes the Hβ emission response to continuum variations in AGN NGC 5548, using photoionization models to understand BLR properties and the observed emission trend.

Contribution

It introduces a constant density cloud model to successfully reproduce the Hβ emission trend and constrains BLR parameters in a prototypical AGN.

Findings

Successfully modeled Hβ shallowing with continuum increase

Constrained BLR cloud density and size consistent with reverberation mapping

Discussed implications of BLR covering factor on emission trends

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.