Multiwavelength study of extreme variability in LEDA 1154204: A changing-look event in a type 1.9 Seyfert

¹ Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland
² Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0424, USA
³ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Dr. Karl Remeis-Observatory & ECAP, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁵ Center for Theoretical Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland
⁶ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
⁷ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁸ Department of Astronomy, Vanderbilt University, 6301 Stevenson Center, Nashville, TN 37235, USA
⁹ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Department of Physics, University of Johannesburg, Kingsway, 2006 Auckland Park, Johannesburg, South Africa
¹² South African Astronomical Observatory, PO Box 9 Observatory, 7935 Cape Town South Africa
¹³ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹⁴ Department of Physics, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa
¹⁵ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du six août 19c, B-4000 Liége (Sart-Tilman), Belgium
¹⁶ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, D-85741 Garching, Germany
¹⁷ Department of Physics & McDonnell Center for the Space Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, MO 63130, USA
¹⁸ Lehrstuhl für Astronomie, Universität Würzburg, Emil-Fischer-Straße 31, 97074 Würzburg, Germany

^⋆ Corresponding author: tathagata@camk.edu.pl

Received: 15 September 2023
Accepted: 1 August 2025

Abstract

Context. Multiwavelength studies of transients in actively accreting supermassive black holes have revealed that large-amplitude variability is frequently linked to significant changes in the optical spectra. This phenomenon is known as a changing-look active galactic nucleus (CLAGN).

Aims. In 2020, the Zwicky Transient Facility detected a transient flaring event in the type 1.9 AGN LEDA 1154204, wherein the brightness sharply increased by 0.55 mag in one month and then began to decay. Spectrum Roentgen Gamma (SRG)/eROSITA also observed the object as part of its all-sky X-ray surveys after the flare had started to decay.

Methods. We performed a three-year multiwavelength follow-up campaign to track the spectral and temporal characteristics of the source during the post-flare fading. This campaign included optical spectroscopy, X-ray spectroscopy and photometry, and ultraviolet, optical, and infrared continuum photometry.

Results. Optical spectra taken near the flare peak revealed a broad double-peaked Hβ emission and a blue continuum, neither of which were detected in a 2005 archival spectrum. The broad Hβ had increased by a factor of > 5–6. From late 2020 through 2023, the broad Balmer-line flux faded as the continuum faded, and the Balmer decrement increased by ∼2.2. This is consistent with the expected ionization response. The X-ray spectrum exhibits no significant spectral variability despite dramatic flux variation of a factor of 17. There is no evidence of a soft X-ray excess, which indicates an energetically unimportant warm corona.

Conclusions. The transient event was likely triggered by a disk instability in a preexisting AGN-like accretion flow that culminated in the observed multiwavelength variability (X-rays via thermal Comptonization, illumination of the broad-line region, and infrared dust echo) and in the CLAGN event.