Changing looks (CL) of the nucleus of Seyfert galaxy NGC 1566 compared with other CL AGNs

We present results of the long-term multi-wavelength study of optical, UV and X-ray variability of the nearby changing-look (CL) Seyfert NGC 1566 observed with the Swift Observatory and the MASTER Global Robotic Network from 2007 to 2020. We started spectral observations with South African Astronomical Observatory 1.9-m telescope soon after the brightening was discovered in July 2018 and present here the data for the interval between Aug. 2018 to Sep. 2019. We summarized previously published  (MNRAS 2019,2020) spectral (SAAO) and photometric results (SWIFT and MASTER), and presented new observations (AN 2022), taking into account most recent versions of the software and calibration files.  In the first publication (MNRAS, 2019), we reported on the change in the spectral type of the object. In the second (MNRAS, 2020) publication we concentrated on the remarkable post-maximum behaviour after July 2018 when the flux was  decreased in all wavelengths (from X-ray to optical) with some fluctuations. We observed four significant re-brightenings in the period after the main maximum. We found differences in X-ray and UV/optical variability. An X-ray flux minimum occurred in Mar. 2019. The UV minimum occurred about 3 months later. It was accompanied by a decrease of the Luv/Lx ratio during 2018-2020 . New post-maximum spectra covering (31 Nov. 2018 - 23 Sep. 2019) show dramatic changes compared to 2 Aug. 2018, with fading of the broad lines and [Fe X] 6374 until Mar. 2019. These lines became somewhat brighter in Aug.-Sep. 2019. Effectively, two CL states were observed for this object: changing to type 1.2 and then returning to the low state as a type 1.8 Sy. We suggest that the changes are due mostly to fluctuations in the energy generation. The estimated Eddington ratios are about 0.055% for minimum in 2014 and 2.8% for maximum in 2018. Variability properties of NGC1566 were compared with our results for other CL AGNs: NGC4151, NGC2617, NGC3516, and Mrk6. We found some common properties in variability of the NGC 1566 with other CL AGNs which can be useful for the future models intended to explain the CL phenomenon