CHANGING LOOKS OF THE NUCLEUS OF THE SEYFERT GALAXY NGC 1566 COMPARED WITH OTHER CHANGING-LOOK AGNS

Abstract

We present results of a long-term optical, UV and X-ray study of variability of the nearby changing-look (CL) Seyfert NGC 1566 which was observed with the Swift Observatory from 2007 to 2020. We summarize our previously published spectroscopic and photometric results and present new observations. We reported on the alteration in the spectral type of NGC 1566 in 2018 ( Paper
I). Moreover, we focused on the exceptional postmaximum behavior after 2018 July, when all bands dropped with some fluctuations (Paper II). We observed four significant re-brightenings in the post-maximum period. We have found differences in X-ray and UV/Optical variability. The Luv=Lx ratio was decreased during 2018-2020. New post-maximum spectra covering the period
2018 November 31 to 2019 September 23 show dramatic changes compared to 2018 August 2, with fading of the broad lines and [Fe X] 6374 until 2019 March (REF2). Effectively, two changing look (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 mostly due to fluctuations in the
energy generation.

Variability properties of NGC1566 are compared with our results for other CL active galactic nuclei (AGNs).