摘要：The discovery of spectral type transition of active galactic nuclei (AGNs), the so-called "changing-look" (CL) phenomenon, challenges the widely accepted AGN paradigm, not only in the orientation-based unified model, but also in the standard disk model.In past decades, only a couple of nearby repeat changing-look active galactic nuclei (CL-AGNs) have been identified.Here we report spectroscopic observations of UGC 3223 over the course of 18 yr, from 2001 onwards.Combining the spectrum taken in 1987 by Stirpe, we have witnessed its type transitions from $1.5\to 2.0\to 1.8$ over 32 yr, and captured a long-lived (at least 10 yr) thorough "turn-off" state with a spectrum typical of a Seyfert 2 galaxy.The long-term thorough turn-off state probably suggests a once-dormant and an awakening central engine in UGC 3223.We argue that the (dis)appearance of the broad Balmer emission lines can be explained by the disk–wind broad-line region model given the evolution of the calculated Eddington ratio of accretion of the supermassive black hole.
