New changing look case in NGC 1566

ATel #11915; V. L. Oknyansky, V. M. Lipunov, E. S. Gorbovskoy (M. V. Lomonosov Moscow State University, Moscow, Russia), H. Winkler, F. van Wyk (University of Johannesburg, South Africa), S. Tsygankov (University of Turku, Finland; IKI, Russia), D. A.H. Buckley (South African Astronomical Observatory, South Africa)
on 5 Aug 2018; 18:01 UT
Credential Certification: Victor Oknyansky (oknyan@mail.ru)

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We report on follow-up optical photometric (MASTER Global Robotic Network) and spectral observations (1.9-m SAAO) of the nearby changing look AGN NGC 1566, which was discovered to be in outburst in hard X-rays by the INTEGRAL observatory on 2018 June 12-19 (Ducci et al., ATel #11754), and was then investigated with the Swift observatory (Kuin et al. ATel #11786, Grupe and Komossa ATel #11903). An ASAS-SN V-band light curve for the period 2014-2018 (Dai et al., ATel #11893) shows that nuclear the outburst became visible in September 2017. The 2014-2018 mid-infrared light curves of the object for show that the nucleus brightened by one magnitude at 3.4 microns and 1.4 magnitudes at 4.6 microns between January 2017 and July 2018 (Cutry et al., ATel #11913). In the past the object spent much time in a low-luminosity state where the broad permitted lines were nearly undetectable (Alloin et al. 1986, ApJ 308, 23). It had several recurrent brightening events when its type changed from Sy 1.9-Sy 1.8 to Sy 1.5-Sy 1.2 states (Phillips et al. 1992, ApJ 308, 23). Our MASTER data include old unfiltered archival data from 2014, new special intensive monitoring in the B,V bands, as well as unfiltered W band data starting in the beginning of July 2018. We show the light curve (here) for unfiltered data reduced to the V-band system of ASAS-SN. As it is seen from the light curve, our magnitudes are in a good agreement with the ASAS-SN V-band light curve for 2014-2018. From our data a slow linear decline after the maximum at the beginning of July is clearly seen. The brightening in the beginning of ASAS-SN light curve in 2014-2015 is due to a supernova (SN), but is not seen in our data as our aperture excluded the SN. Analysis of the Swift/XRT data demonstrates a substantial increase of flux by 1.5 orders of magnitude accompanying the brightening in the UV and optical bands (here). Low resolution optical spectra were obtained on 2 August 2018 with the SAAO 1.9-m telescope. The spectra reveal a dramatic strengthening of the broad emission lines compared to past published ones. This confirms NGC 1566 to be a changing look Seyfert galaxy. Furthermore: 1. H-beta is quite a lot brighter than [OIII]5007A. The H-beta to N1 line ratio is about 2-2.5, corresponding to a Sy1.2 classification. H-beta has not been observed at this strength before in this object. 2. The HeII4686A is bright and broad. No such strong HeII was visible in the past published spectra (see e.g. Kriss et al., 1991, ApJ 377, L13; Winkler 1992, MNRAS 257, 677). 3. The [FeX]6374A coronal emission line is stronger than [OI]6300A, something that has not been seen before in NGC 1566. We are going to present more details in our next publications.