Exploring Variation of Double-peak Broad-line Profile in Strongly Variable AGNs

Abstract The geometry and kinematics of the broad-line region (BLR) in active galactic nuclei (AGNs) are still unclear, which is crucial for studying the physics and evolution of supermassive black holes 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 and 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 ≳ 5 Z ⊙ is required to reproduce the observational anticorrelation 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.