2023MNRAS.526.1588Z

Optical quasi-periodic oscillations (QPOs) are the most preferred signs of sub-pc binary black hole (BBH) systems in active galactic nucleus (AGN). In this manuscript, robust optical QPOs are reported in quasar SDSS J1609+1756 at z = 0.347. In order to detect reliable optical QPOs, four different methods are applied to analyse the 4.45 yr-long Zwicky Transient Facility (ZTF) g/r/i-band light curves of SDSS J1609+1756, direct fitting results by sine function, generalized Lomb-Scargle periodogram, autocross correlation function, and weighted wavelet Z-transform method. The four different methods can lead to well determined reliable optical QPOs with periodicities ∼340 d with confidence levels higher than 5σ, to guarantee the robustness of the optical QPOs in SDSS J1609+1756. Meanwhile, based on simulated light curves through continuous autoregressive process to trace intrinsic AGN activities, confidence level higher than 3σ can be confirmed that the optical QPOs are not misdetected in intrinsic AGN activities, reconfirming the robust optical QPOs and strongly indicating a central sub-pc BBH system in SDSS J1609+1756. Furthermore, based on apparent redshifted shoulders in broad Balmer emission lines in SDSS J1609+1756, space separation of the expected central BBH system can be estimated to be smaller than 107 ± 60 light-days, accepted upper limit of total BH mass ∼(1.03 ± 0.22) × 10⁸ M_⊙. Therefore, to detect and report BBH system expected optical QPOs with periodicities around 1 yr is efficiently practicable through ZTF light curves, and combining with peculiar broad-line emission features, further clues should be given on space separations of BBH systems in broad-line AGN in the near future.