Exploring variation of double-peak broad-line profile in strongly variable AGNs

TL;DR

This study models the evolution of broad-line profiles in variable AGNs, revealing how line shapes change with Eddington ratio and metallicity, and emphasizing the importance of long-term monitoring for understanding BLR physics.

Contribution

It introduces a model linking BLR line profile variations to Eddington ratio and metallicity, providing new insights into AGN variability and BLR structure.

Findings

Double-peak profiles at low Eddington ratios

Single-peak profiles at high Eddington ratios

High metallicity needed to match observations

Abstract

The geometry and kinematics of the broad-line region (BLR) in AGNs are still unclear, which is crucial for studying the physics and evolution of supermassive black holes (SMBHs) and AGNs. The broad-line profile provides valuable information on BLR geometry and kinematics. In this work, we explore the evolution of line profiles in variable AGNs based on the BLR model of Czerny \& Hryniewicz, where the BLR is driven by the radiation pressure acting on dust at the surface layers of the accretion disk. The line profiles in the low-Eddington-ratio regime show a double-peak profile, which will become a single peak at high Eddington ratios. The high metallicity of $Z\gtrsim 5Z_{\odot}$ is required to reproduce the observational anti-correlation between the peak separation of broad lines and the Eddington ratio for a sample of AGNs. For the broad lines in variable AGNs, it will take several years to several decades to change their line profile if the disk luminosity suffers strong variation in a much shorter timescale. More monitoring of the broad line and continuum in strongly variable AGNs can shed special light on BLR physics.