Changing-look behaviour and amplitude-modulated QPO in blazar Ton 599

ABSTRACT Active galactic nuclei (AGNs) are powered by supermassive black holes and exhibit different observational types, primarily due to orientation effects according to the unified model of AGN. Changing-look AGNs (CL AGNs), however, challenge this paradigm by transitioning between different types over time. A hallmark of CL AGNs is the variability of the broad emission line (BEL) flux relative to the continuum. In this study, we investigate the optical spectral evolution of Ton 599 and its correlation with the $\gamma$-ray flux in the GeV band. The optical continuum flux density exhibits strong variability, with a normalized excess variance of $F_{\rm var} = 1.089$. In contrast, the MgII flux varies with a much smaller amplitude ($F_{\rm var} = 0.194$), while the equivalent width of Mg ii shows intermediate variability ($F_{\rm var} = 0.686$). A strong correlation is observed between the optical continuum and the $\gamma$-ray flux, suggesting a common origin. In addition, an amplitude-modulated quasi-periodic oscillation (QPO) with a period of $\sim$176 d is identified in the $\gamma$-ray light curve during MJD 57000–58400, with modulation on a $\sim$400-d time-scale. A similar QPO with a period of $\sim$181 d is also detected in the optical g-band light curve during MJD 59500–60100. The presence of these QPOs in both bands likely reflect the influence of geometrical structures in the jet as well as long-term bursts. Our results suggest that jet dynamics may drive CL events in some AGNs, contributing to our understanding of the physical processes governing AGN jets and offering critical refinements to the unified model.