The ePESSTO+ census of ZTF nuclear transients. I. What flares in galactic nuclei?

Abstract

Transients in galactic nuclei arise from distinct processes, only some of which trace the supermassive black hole (SMBH). These include tidal disruption events (TDEs), changing-look active galactic nuclei, Bowen fluorescence flares (BFFs) and ambiguous nuclear transients, alongside supernovae that appear nuclear but are unrelated to the SMBH. Reliable classification requires spectroscopy, which will not scale to the alert volumes of the Legacy Survey of Space and Time (LSST). We aim to characterise the population of nuclear transients, to identify early observables capable of distinguishing SMBH-related flares, and TDEs in particular, from the dominant supernova background, and to assess the biases this selection introduces. We implemented a real-time filter on the Lasair broker, selecting transients from the Zwicky Transient Facility (ZTF) alert stream by nuclear coincidence with the host galaxy, with no photometric cuts, to retain SMBH-related flares in both quiescent and active hosts. The filter ran over 16 months, with candidates reported for classification through ePESSTO+ and other facilities. The filter returned 476 candidates, of which 202 were spectroscopically classified, yielding predominantly supernovae, 10 confirmed TDEs (5%) and one BFF. The mean g-r colour and the rise time separate TDEs from the supernova background. Our filter recovers 10 of the 25 TDEs classified in the same period and declination range; most misses are technical or observational rather than physical. Simple real-time filtering combined with basic photometric criteria can already isolate TDEs efficiently from nuclear transient alert streams, and high completeness is achievable. In the LSST era, where samples will be predominantly photometric, selection bias will become as important as contamination, and retaining flares in active hosts is necessary if unusual events are not to be excluded.