The complex periodic variability of PKS 0301-243: hints of a supermassive black hole binary candidate
In hierarchical galaxy formation, supermassive black hole binaries are a ubiquitous consequence of frequent galaxy mergers, yet subparsec binaries remain difficult to be confirmed. Periodic flux modulations may arise from the orbital motion of black holes in close binaries, while relativistic jet precession can provide an additional source of periodic variability. Such periodic signatures therefore motivate us to identify close binary candidates. Here, we report an optical periodicity of $269^{+13}_{-14}$ days in both Zwicky Transient Facility g and r bands of the blazar PKS 0301-243, with corrected global significances of $2.75σ$ and $2.18σ$, respectively. This signal persists over 7 cycles across a baseline of ~2000 days. We interpret this period as a possible modulation associated with the orbital motion of a supermassive black hole binary. A longer optical periodicity of $889^{+47}_{-46}$ days is identified in Katzman Automatic Imaging Telescope, with corrected global significance of $1.97σ$. The longer periodicity spans approximately 5 cycles over a baseline of ~4500 days, potentially associated with jet precession. Both periodicities are independently identified using multiple methods. A supermassive black hole binary incorporating a relativistic jet and a circumbinary disc could reproduce the observed light curve ensemble. For a total mass of $10^{8.95} M_\odot$ and a mass ratio q = 0.1, the derived milli-parsec separation and a short gravitational wave driven coalescence time, $t_c = 1122^{+144}_{-144}$ years, imply that PKS 0301-243 may be a rare source of nano-hertz gravitational waves and a potential multimessenger laboratory for the terminal stages of supermassive black hole coalescence.