On type 1 active galactic nuclei with double-peaked [O~{\sc iii}]. II. properties of long-term optical variability

TL;DR

This study investigates whether double-peaked [O III] profiles in type 1 AGNs indicate dual AGN systems by analyzing long-term optical variability and finds little evidence supporting this connection.

Contribution

It provides the first systematic analysis of optical variability in double-peaked [O III] AGNs, challenging the assumption that these features reliably indicate dual AGNs.

Findings

No significant difference in variability between double-peaked AGNs and normal AGNs.

Simulations show dual AGNs tend to have lower variability amplitudes.

Dual AGN fraction estimated at around 3%, suggesting double-peaked [O III] is not a reliable dual AGN indicator.

Abstract

Double-peaked \oiii~profiles could potentially indicate kiloparsec-scale dual AGNs. We analyze long-term optical light curves of 35 type 1 AGNs with such features from our recent catalog in Zheng et al. (2025). These light curves are obtained from the Catalina Sky Survey and modeled using a Damped Random Walk (DRW) process. A control sample of 210 normal type 1 AGNs matched in redshift, intrinsic luminosity, and black hole mass is also studied. If the double-peaked \oiii~are caused by two type 1 AGNs (dual type 1 AGN), then the combined variability from the two AGNs would be expected to differ from that of a single type 1 AGN. However, there is no statistically significant difference in the variability timescale $\tau$ and intrinsic variability amplitude $\sigma$ between these double-peaked AGNs and the control sample of 210 normal type 1 AGNs. Crucially, computer simulations reveal that dual AGN systems systematically produce lower variability amplitudes than single AGNs, which is inconsistent with the observed variability properties of our double-peaked \oiii~sample. Moreover, simulations suggest that the fraction of dual type 1 AGNs is $\sim$ 3\%, indicating that double-peaked \oiii~may not be a reliable indicator of dual type 1 AGNs in these systems. However, this does not rule out the possibility that some objects may still host dual AGNs involving other combinations, such as type 1+type 2 AGNs. Future studies with larger samples and higher-quality light curves will help clarify the true nature of these systems.