Back from the dead: AT2019aalc as a candidate repeating tidal disruption event in an active galactic nucleus

¹ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB) Universitätsstraße 150 44801 Bochum, Germany
² Ruhr Astroparticle and Plasma Physics Center (RAPP Center) 44801 Bochum, Germany
³ Leiden Observatory, Leiden University Postbus 9513 2300 RA Leiden, The Netherlands
⁴ Deutsches Elektronen-Synchrotron (DESY) Platanenallee 6 D-15378 Zeuthen, Germany
⁵ Institut fur Physik, Humboldt-Universität zu Berlin D-12489 Berlin, Germany
⁶ Division of Physics, Mathematics, and Astronomy, California Institute of Technology Pasadena CA 91125, USA
⁷ University of Maryland, Department of Astronomy 4296 Stadium Dr College Park MD 20742, USA
⁸ NASA Goddard Space Flight Center, Astrophysics Science Division 8800 Greenbelt Rd Greenbelt MD 20771, USA
⁹ Center for Research and Exploration in Space Science and Technology, NASA/GSFC Greenbelt MD 20771, USA
¹⁰ Department of Particle Physics and Astrophysics, Weizmann Institute of Science 76100 Rehovot, Israel
¹¹ Cahill Center for Astrophysics, California Institute of Technology, MC 249-17 1200 E California Boulevard Pasadena CA 91125, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 August 2024
Accepted: 21 November 2025

Abstract

Context. To date, three nuclear transients have been associated with high-energy neutrino events. These transients are generally thought to be powered by tidal disruption events (TDEs) in stars caused by massive black holes. However, AT2019aalc, hosted in a Seyfert-1 galaxy, has not yet been classified due to a lack of multiwavelength observations. Interestingly, the source re-brightened 4 years after its discovery.

Aims. Our aim is to constrain the physical mechanism responsible for the second optical flare, which may also provide clues to the origin of the initial event.

Methods. We conducted a multiwavelength monitoring program (from radio to X-rays) of AT2019aalc during its re-brightening in 2023–2024.

Results. The observations revealed multiple X-ray flares during the second optical flaring episode of the transient and a uniquely bright UV counterpart. The second flare, similar to the first one, is accompanied by IR dust echo emission. A long-term radio flare was found with an inverted spectrum. Optical spectroscopic observations revealed the presence of Bowen fluorescence lines and strong high-ionization coronal lines, indicating an extreme level of ionization in the system.

Conclusions. The results suggest that the transient can be classified as a Bowen fluorescence flare (BFF), a relatively new sub-class of flaring active galactic nuclei (AGNs). AT2019aalc can be also classified as an extreme coronal line emitter (ECLE). We find that in addition to AT2019aalc, another BFF, AT2021loi, is spatially coincident with a high-energy neutrino event. We propose a repeating TDE scenario within an AGN framework to explain the multiwavelength properties of AT2019aalc and suggest a possible connection among ECLEs, BFFs, and TDEs occurring in AGNs.