The transient event in NGC 1566 from 2017 to 2019

I. An eccentric accretion disk and a turbulent, disk-dominated broad-line region unveiled by double-peaked Ca II and O I lines^⋆

¹ Institut für Astrophysik und Geophysik, Universität Göttingen, Friedrich-Hund Platz 1, 37077 Göttingen, Germany
e-mail: martin.ochmann@uni-goettingen.de
² Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
³ South African Astronomical Observatory, PO Box 9 Observatory Road, Observatory, 7935 Cape Town, South Africa
⁴ Southern African Large Telescope, PO Box 9 Observatory, 7935 Cape Town, South Africa
⁵ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁶ Department of Physics, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa
⁷ Department of Physics, Faculty of Natural Sciences, University of Haifa, Haifa 3498838, Israel
⁸ Haifa Research Center for Theoretical Physics and Astrophysics, University of Haifa, Haifa 3498838, Israel
⁹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
¹⁰ Universidad Católica del Norte, Instituto de Astronomía, Avenida Angamos 0610, Antofagasta, Chile
¹¹ Department of Physics, Geology, and Engineering Technology, Northern Kentucky University, 1 Nunn Drive, Highland Heights, KY 41099, USA
¹² School of Physics & Astronomy and the Wise Observatory, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel
¹³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁴ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
¹⁵ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Villanueva de la Cañada, 28691 Madrid, Spain

Received: 10 November 2023
Accepted: 16 February 2024

Abstract

Context. NGC 1566 is a local face-on Seyfert galaxy and is known for exhibiting recurrent outbursts that are accompanied by changes in spectral type. The most recent transient event occurred from 2017 to 2019 and was reported to be accompanied by a change in Seyfert classification from Seyfert 1.8 to Seyfert 1.2.

Aims. We aim to study the transient event in detail by analyzing the variations in the optical broad-line profiles. In particular, we intend to determine the structure and kinematics of the broad-line region.

Methods. We analyzed data from an optical spectroscopic variability campaign of NGC 1566 taken with the 9.2 m Southern African Large Telescope (SALT) between July 2018 and October 2019 triggered by the detection of hard X-ray emission in June 2018. We supplemented this data set with optical to near-infrared (NIR) spectroscopic archival data taken by VLT/MUSE in September 2015 and October 2017, and investigated the emission from different line species during the event.

Results. NGC 1566 exhibits pronounced spectral changes during the transient event. We observe the emergence and fading of a strong power-law-like blue continuum as well as strong variations in the Balmer, He I, and He II lines and the coronal lines [Fe VII], [Fe X], and [Fe XI]. Moreover, we detect broad double-peaked emission line profiles of O Iλ8446 and the Ca IIλλ8498, 8542, 8662 triplet. This is the first time that genuine double-peaked O Iλ8446 and Ca IIλλ8498, 8542, 8662 emission in AGN is reported in the literature. All broad lines show a clear redward asymmetry with respect to their central wavelength and we find indications for a significant blueward drift of the total line profiles during the transient event. The profiles and the FWHM of the Balmer lines remain largely constant during all observations. We show that the double-peaked emission line profiles are well approximated by emission from a low-inclination, relativistic eccentric accretion disk, and that single-peaked profiles can be obtained by broadening due to scale-height-dependent turbulence. Small-scale features in the O I and Ca II lines suggest the presence of inhomogeneities in the broad-line region.

Conclusions. We conclude that the broad-line region in NGC 1566 is dominated by the kinematics of a relativistic eccentric accretion disk. The broad-line region can be modeled to be vertically stratified with respect to scale-height turbulence with O I and Ca II being emitted close to the disk in a region with high (column) density, while the Balmer and helium lines are emitted at greater scale height above the disk. The observed blueward drift might be attributed to a low-optical-depth wind launched during the transient event. Except for this wind, the observed kinematics of the broad-line region remain largely unchanged during the transient event.