Long-term Optical Spectroscopic Variability of BL Lacertae Object AO 0235+164

Abstract The changing-look (CL) phenomenon in active galactic nuclei (AGNs) poses a significant challenge to the unified model. As a special subclass of AGN, blazars provide valuable insights. The CL behavior in the blazars not only helps us probe the intrinsic physical properties of AGN structures but also offers an ideal laboratory for exploring the dynamical coupling between the accretion disk and relativistic jets. Compiled with the historical spectroscopic data, together with an analysis of the γ -ray and optical light curves, we find the following: (1) AO 0235+164 exhibits an oscillatory transitions between BL Lac and FSRQ states. That is, when the accretion rate increases, jet activity gradually weakens and the source transitions to an FSRQ state, and when the accretion rate decreases, the jet re-enters a flaring phase and the source returns to a BL Lac state. (2) The positive correlation relationship between equivalent width and accretion rate is primarily observed during the FSRQ phase, indicating that the broad emission lines in this stage are mainly driven by accretion-disk radiation. These results suggest that the changing-look behavior of AO 0235+164 cannot be explained solely by continuum dilution, but is instead regulated by the coupled evolution of jet activity and accretion-disk emission. We argue that strong evidence for the CL blazar represent a transitional population with physical properties intermediate between classical FSRQ and BL Lac object could be obtained from long-term optical spectroscopic observation.