Investigating changing-look active galactic nuclei with long-term optical and X-ray observations

Context. Broad emission lines in the UV/optical spectra of changing-look active galactic nuclei (CLAGNs) appear and disappear on timescales of months to decades. Aims. We investigate how changing-look (CL) transitions depend on several active galactic nucleus (AGN) parameters, such as the accretion rate, obscuration properties, and black hole mass. Methods. We studied a sample of 20 nearby optically identified CLAGNs from the BAT AGN Spectroscopic Survey (BASS) using quasi-simultaneous optical and X-ray observations taken in the last ∼40 years. Results. We find that for all CLAGNs, the transition is accompanied by a change in the Eddington ratio. The CL transitions are not associated with changes in the obscuration properties of the AGNs. CLAGNs are found to have a median Eddington ratio lower than that of the AGNs in the BASS sample in which CL transitions were not detected. The median transition Eddington ratio (the Eddington ratio at which an AGN changes its state) is found to be ∼0.01 for type 1 ↔ 1.8, 1.9, and 2 transitions, which is consistent with the hard ↔ soft state transition in black hole X-ray binaries. Most CL events are constrained to have occurred within 3–4 years, which is considerably shorter than the expected viscous timescale in AGN accretion disks. Conclusions. The transitions of the optical CLAGNs studied here are likely associated with state changes in the accretion flow, possibly driven by disk instability.