Still alive and kicking: A significant outburst in changing-look AGN Mrk 1018

¹ Leibniz-Institut für Astrophysik (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: rbrogan@aip.de
² University of Potsdam, Institute of Physics and Astronomy, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany
³ EUMETSAT, Eumetsat Allee 1, 64295 Darmstadt, Germany
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁶ School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW 2109, Australia
⁷ Astronomy, Astrophysics and Astrophotonics Research Centre, Macquarie University, Sydney, NSW 2109, Australia
⁸ Centre for Astrophysics and Supercomputing, School of Science, Swinburne University of Technology, Hawthorn, VIC 3122, Australia
⁹ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Mount Stromlo Rd, Stromlo, ACT 2611, Australia
¹⁰ Sydney Institute for Astronomy, School of Physics, University of Sydney, Sydney, NSW 2006, Australia
¹¹ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL University, Sorbonne University, 75014 Paris, France
¹² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹³ Facultad de Ciencias, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
¹⁴ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
¹⁵ Department of Astronomy, University of California, San Diego, CA 92092, USA
¹⁶ School of Mathematics and Physics, The University of Queensland, St Lucia, QLD 4072, Australia
¹⁷ Department of Physics and Astronomy, University of Manitoba, Winnipeg MB R3T 2N2, Canada

Received: 21 March 2023
Accepted: 19 July 2023

Abstract

Context. Changing-look active galactic nuclei (AGN) have been observed to change their optical spectral type. Mrk 1018 is particularly unique: first classified as a type 1.9 Seyfert galaxy, it transitioned to being a type 1 Seyfert galaxy a few years later before returning to its initial classification as a type 1.9 Seyfert galaxy after ∼30 years.

Aims. We present the results of a high-cadence optical monitoring programme that caught a major outburst in 2020. Due to sunblock, only the decline could be observed for ∼200 days. We studied X-ray, UV, optical, and infrared data before and after the outburst to investigate the responses of the AGN structures.

Methods. We derived a u′-band light curve of the AGN contribution alone. The flux increased by a factor of ∼13. We confirmed this in other optical bands and determined the shape and speed of the decline in each waveband. The shapes of Hβ and Hα were analysed before and after the event. Two XMM-Newton observations (X-ray and UV) from before and after the outburst were also exploited.

Results. The outburst is asymmetric, with a swifter rise than decline. The decline is best fit by a linear function, ruling out a tidal disruption event. The optical spectrum shows no change approximately eight months before and 17 months after. The UV flux is increased slightly after the outburst but the X-ray primary flux is unchanged. However, the 6.4 keV iron line has doubled in strength. Infrared data taken 13 days after the observed optical peak already show an increased emission level as well.

Conclusions. Calculating the distance of the broad-line region and inner edge of the torus from the supermassive black hole can explain the multi-wavelength response to the outburst, in particular: i) the unchanged Hβ and Hα lines, ii) the unchanged primary X-ray spectral components, iii) the rapid and extended infrared response, as well as iv) the enhanced emission of the reflected 6.4 keV line. The outburst was due to a dramatic and short-lasting change in the intrinsic accretion rate. We discuss different models as potential causes.