Back from the dead: AT2019aalc as a candidate repeating TDE in an AGN

TL;DR

This work presents a comprehensive multi-wavelength investigation of AT2019aalc during its 2023/2024 re-brightening, revealing two luminous optical flares, persistent Bowen fluorescence lines, and an extreme coronal line-emitting environment within a Seyfert-1 host. The campaign uncovers soft X-ray flares that precede optical/UV bumps, a long-term inverted-spectrum radio flare with a compact VLBI core, and infrared dust echoes tied to both flares, all of which support a Bowen Fluorescence Flare (BFF) classification and an Extreme Coronal Line Emitter (ECLE) nature. The authors propose a repeating TDE scenario inside an AGN to explain the complex light curves and spectral evolution, with possible connections to other BFFs and TDEs in AGN environments, and they discuss potential links to IceCube neutrino events. Altogether, AT2019aalc provides strong evidence that TDEs can occur in active nuclei and may contribute to high-energy neutrino production through outflows and disk–debris interactions, highlighting a path toward identifying similar multi-messenger transients in the AGN population.

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 disruptions of stars (TDEs) by massive black holes. However, AT2019aalc, hosted in a Seyfert-1 galaxy, was not yet classified due to a lack of multiwavelength observations. Interestingly, the source has 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 multi-wavelength 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, similarly to the first one, is also accompanied by IR dust echo emission. A long-term radio flare is found with an inverted spectrum. Optical spectroscopic observations reveal the presence of Bowen Fluorescence lines and strong high-ionization coronal lines indicating an extreme level of ionization in the system. Conslusions. 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 (AGN). AT2019aalc can be also classified as an extreme coronal line emitter (ECLE). We found 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 multi-wavelength properties of AT2019aalc and suggest a possible connection among ECLEs, BFFs, and TDEs occurring in AGNs.

Figures (19)

• Figure 1: Pan-STARRS i-band pre-flare image centered on the host galaxy position of the transient event AT2019aalc. The object located to the west of the galaxy core is a foreground star. The green contours represent the pre-flare FIRST radio observations taken at $1.4$ GHz. The lowest contour level is drawn at $3\sigma$ image noise level corresponding to $0.4\textrm{\,mJy\,beam}^{-1}$. Additional positive contour levels increase by a factor of two. The peak brightness is $5.8\textrm{\,mJy\,beam}^{-1}$. The red circle points to the Swift-XRT $0.3-10$ keV astrometrically-corrected position of AT2019aalc, determined using the Swift-XRT data products generator astro_swift. Its size indicates the $90\%$ confidence positional error.
• Figure 2: Optical light curve of AT2019aalc derived from ZTF difference photometry. The transparent markers indicate negative flux i.e. the flux decreased below the mean flux of the reference images. Two distinct flares can be seen. The second flare peaked four years after the first one. The blue vertical line indicates the arrival time of the neutrino event IC191119A associated with the transient.
• Figure 3: Multi-wavelength (bottom plot: X-ray and extinction-corrected and host-subtracted optical/UV, middle plot: radio and upper plot: extinction-corrected and host-subtracted IR) light curves of AT2019aalc. Here we show only the positive ZTF fluxes based on the forced photometry results. The dashed gray vertical line indicates the IceCube detection of the high-energy neutrino event IC191119A. The short, purple vertical lines on the top of the bottom panel indicate the peak times of the X-ray flares while the blue ones show the times the main optical $r$-band flare and later the bumps. The grey 'S' letters represent the observing times of the optical spectra..
• Figure 4: Pre-flare long-term optical light curves of the AGN hosting AT2019aalc, starting approximately $14$ years before the discovery of the transient in January $2019$, which is indicated with a blue vertical line.
• Figure 5: The long-term IR variability of AT2019aalc. The W1$-$W2 color indicates heated dust as the dominant origin of the IR emission. The second optical flare is accompanied by another IR flaring episode with color change towards the blue. The plotted magnitudes are in the Vega system and have been corrected for extinction but not for host-galaxy contribution, allowing the full evolution between the flares to be visible.