Modeling the Reverberation Response of the Broad Line Region in Active Galactic Nuclei II: Incorporating Photoionization Models

TL;DR

This paper enhances a reverberation mapping model for active galactic nuclei by integrating photoionization calculations, revealing complex BLR responses and implications for SMBH mass estimates.

Contribution

It introduces a new version of BELMAC that incorporates photoionization models to better simulate BLR line responses to continuum variations.

Findings

Line responses indicate luminosity-weighted radii accurately in many cases.

Negative responses can lead to overestimating BLR radii.

Model-derived SMBH masses can significantly differ from true masses depending on geometry.

Abstract

The broad emission lines (BELs) emitted by Active Galactic Nuclei respond to variations in the ionizing continuum emission from the accretion disk surrounding the central supermassive black hole (SMBH). This reverberation response provides insights into the structure and dynamics of the Broad Line Region (BLR). In Rosborough et al., 2024, we introduced a new forward-modeling tool, the Broad Emission Line MApping Code (BELMAC), which simulates the velocity-resolved reverberation response of the BLR to an input light curve. In this work, we describe a new version of BELMAC, which uses photoionization models to calculate the cloud luminosities for selected BELs. We investigated the reverberation responses of H$\alpha$, H$\beta$, MgII$\lambda$2800 and CIV$\lambda$1550 for models representing a disk-like BLR with Keplerian rotation, radiatively driven outflows, and inflows. The line responses generally provide a good indication of the respective luminosity-weighted radii. However, there are situations when the BLR exhibits a negative response to the driving continuum, causing overestimates of the luminosity-weighted radius. The virial mass derived from the models can differ dramatically from the actual SMBH mass, depending mainly on the disk inclination and velocity field. In single zone models, the BELs exhibit similar responses and profile shapes; two-zone models, such as a Keplerian disk and a biconical outflow, can reproduce observed differences between high- and low-ionization lines. Radial flows produce asymmetric line profile shapes due to both anisotropic cloud emission and electron scattering in an inter-cloud medium. These competing attenuation effects complicate the interpretation of profile asymmetries.