New changing look case in NGC 1566

TL;DR

This study documents a significant changing look event in NGC 1566, showing correlated optical, UV, and X-ray variability, with spectral changes confirming its classification as a changing look Seyfert galaxy.

Contribution

First comprehensive multi-wavelength observational analysis of the changing look phenomenon in NGC 1566 over a decade.

Findings

X-ray flux increased by 1.5 orders of magnitude during brightening

Strongest variability observed in X-ray band, decreasing with wavelength

Detection of high-ionization [FeX] emission lines during the event

Abstract

We present a study of optical, UV and X-ray light curves of the nearby changing look active galactic nucleus in the galaxy NGC 1566 obtained with the Neil Gehrels Swift Observatory and the MASTER Global Robotic Network over the period 2007 - 2018. We also report on our optical spectroscopy at the South African Astronomical Observatory with the 1.9-m telescope on the night 2018 August 2-3. A substantial increase in X-ray flux by 1.5 orders of magnitude was observed following the brightening in the UV and optical bands during the last year. After a maximum was reached at the beginning of 2018 July the fluxes in all bands decreased with some fluctuations. The amplitude of the flux variability is strongest in the X-ray band and decreases with increasing wavelength. Low-resolution spectra reveal a dramatic strengthening of the broad emission as well as high-ionization [FeX]6374 A lines. These lines were not detected so strongly in the past published spectra. The change in the type of the optical spectrum was accompanied by a significant change in the X-ray spectrum. All these facts confirm NGC 1566 to be a changing look Seyfert galaxy.