Lost and Found - A gallery of overlooked optical nuclear transients from the ZTF archive

TL;DR

This study leverages the Fink alert broker to develop an early TDE-detection tool for ZTF data and applies it to archival and development-phase transients, uncovering 19 optical nuclear transients. The sample includes 9 TDE-like events in passive hosts, 8 in active hosts (ANT/ENT candidates), two with uncertain hosts, two newly identified repeated TDEs, and several supernova contaminants, illustrating the rich diversity of nuclear transients and the difficulty of photometric-only classification. By combining phenomenological and physically motivated fits (including $t$de ext{-}fallback models and Bayesian Redback inference) with multi-wavelength archival data, the work highlights both the detection potential and current classifier limitations, and emphasizes the need for multi-wavelength follow-up and careful handling of long-duration and repeated events for future surveys like LSST. The results underscore the importance of archival data in revealing overlooked transients, inform classifier design to balance completeness and purity, and advocate for cross-band alert systems to improve real-time identification and interpretation of nuclear transients.

Abstract

Tidal disruption events (TDEs) correspond to the destruction of a star by the tidal forces around a black hole, leading to outbursts which can last from months to years. These transients are rare, and increasing the current sample is paramount to understand them. As part of the Fink alert broker, we have developed an early detection system for TDEs for the Zwicky Transient Facility (ZTF) data. In this paper, we report on the optical transients we found either during the development of this tool, or when applying the classifier to the existing archive. We use this sample to anticipate what improvements to the TDE detection systems will need to be implemented for future surveys. For all the transients, we present optical and infrared archival photometry from ZTF, WISE, and Catalina, and assess the previous nuclear activity of the host. We fit the ZTF lightcurves with both a phenomenological and a physically-motivated model. We report on a total of 19 optical nuclear transients, out of which nine are in passive galaxies, eight in active galaxies, and two for which the activity of the host is uncertain. Two transients are newly discovered repeated TDE candidates, and we compare them to the current sample of repeated optical nuclear transients. One transient is exceptionally long-lived (over 5 years), in an until-now passive galaxy. Three of the TDE-like flares in active galaxies have absolute g-band magnitudes brighter than -24, making them new Extreme Nuclear Transient (ENT) candidates. One seemingly repeated object was revealed to be two independent supernovae in the same galaxy. This sample shows both the potential of our detection system for future discovery, and the relevance of archival searches to reveal overlooked transients. It also raises several points of concern and avenues of improvement for current and future classifiers.

Figures (14)

• Figure 1: Representation of the timescale and peak absolute magnitude of the various optical transients considered in this paper. The exact borders of each category are generally not well-defined -- in particular, there might be a continuum between ANTs and ENTs, the latter being for now crudely defined as brighter than absolute magnitude -24. The orange boxes correspond to supernovae events, and the blue ones to accretion-powered events (the interpretation of FBOTs is still open). The clear overlaps between categories indicate the difficulty of classifying these events based on photometric data only. This figure is inspired from Fig. 4 of hinkle_extreme_2024.
• Figure 2: Comparison of the behaviors of ZTF19acnskyy ("Ansky", top panel) and AT2020ukj (bottom panel), showing their ZTF r and g bands (orange and dark blue), and the Swift/XRT detections and upper limits (light blue circles and downwards triangles). This shows their similarities in terms of long decay timescales, as well as the complex late-time X-ray behavior of Ansky.
• Figure 3: ZTF forced photometry lightcurve of AT2023adr, a candidate repeated TDE. Points with a signal to noise ratio below 3 have been plotted in transparence. The full line corresponds to a double TDE profile (gaussian rise, $\propto t^{-5/3}$ decay) fitted to the $g$-band lightcurve. The dotted vertical line shows the timing of the ePESSTO+ optical spectrum, during the second peak.
• Figure 4: Evidence for late-time plateau in AT2020pno, with the data and the fitted models in the top panel, and the corresponding residuals in the bottom three panels. Here three models are compared: a simple $\propto t^{-5/3}$ decay (orange, significant negative then positive residuals), a constant plateau (dark blue, slight negative then positive residuals), and a linearly decreasing plateau (light blue, no strong residuals). Data here is shown in bins of 15 days.
• Figure 5: Evidence for two TDE-like flares 16 years apart in ZTF23abjvojy. Its lightcurve (top panel) is shown for both CRTS V-band (light blue) and ZTF g-band (dark blue) and r-band (orange) data. We also provide comparison with two other known repeated AGN flares: AT2019aalc (middle panel) and AT2021aeuk (botton panel). The transparent points correspond to unbinned data, and the opaque points to data binned in bins of 15 days. We draw attention to the fact that the bands for CRTS and ZTF here are not the same and not cross-calibrated, so their respective magnitudes should not be directly compared.