Letter to the Editor

Quasi-periodic oscillations in optical color evolutions to support sub-parsec binary black hole systems in broad line active galactic nuclei

Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, GuangXi University, No. 100, Daxue Road, Nanning 530004, PR China

^⋆ Corresponding author: xgzhang@gxu.edu.cn

Received: 9 September 2025
Accepted: 15 October 2025

Abstract

Optical quasi-periodic oscillations (QPOs) with periodicities that range from hundreds to thousands of days have been accepted as an efficient indicator for sub-parsec (sub-pc) binary black hole systems (BBHs) in broad line active galactic nuclei (BLAGN). However, considering the intrinsic variability (red noise) of BLAGN, it is still an open question on the physical origin of detected optical QPOs from AGN variability or truly from sub-pc BBHs. Here, a simple method is proposed to support optical QPOs related to sub-pc BBHs by detecting QPOs in time-dependent optical color evolutions of BLAGN. Periodic variations of obscurations are expected on optical light curves related to sub-pc BBHs, but there should be non-periodic variable obscurations on optical light curves in normal BLAGN. Through simulated optical light curves for intrinsic AGN variability by continuous autoregressive processes with time durations of around 2800 days (similar to the time durations of light curves in the ZTF), the probability is definitely lower than 3.3 × 10⁻⁷ that QPOs can be detected in the corresponding optical color evolutions, but about 3.1 × 10⁻² that optical QPOs with periodicity shorter than 1400 days (at least two cycles) can be detected in the simulated single-band light curves. Therefore, the confidence level is definitely higher than 5σ to support the QPOs in color evolutions not related to intrinsic AGN variability, but truly related to sub-pc BBHs. In the near future, the proposed method can be applied to searching for reliable optical QPOs in BLAGN through multi-band light curves from the ZTF and the upcoming LSST.