The post-maximum behaviour of the changing-look Seyfert galaxy NGC 1566

TL;DR

This study tracks the long-term optical, UV, and X-ray variability of the changing-look Seyfert galaxy NGC 1566, revealing multiple re-brightenings and spectral changes that suggest fluctuations in energy generation drive its state transitions.

Contribution

It provides the first detailed multi-wavelength analysis of NGC 1566's post-maximum behaviour, capturing multiple re-brightenings and spectral state changes over a decade.

Findings

Three significant re-brightenings observed post-2018

Spectral lines faded and then brightened, indicating changing states

Eddington ratios varied from 0.055% to 2.8% during the study

Abstract

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 2019. 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. This paper concentrates on the remarkable post-maximum behaviour after July 2018 when all bands decreased with some fluctuations. We observed three significant re-brightenings in the post-maximum period during 17 Nov. 2018 - 10 Jan. 2019, 29 Apr. - 19 Jun. 2019 and 27 Jul.- 6 Aug. 2019. 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. 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.