Precursors in tidal disruption events: Repeating, fast, and active galactic nucleus-hosted TDEs

Tidal disruption events (TDEs) are rare transients that provide important insights into the physics of galactic nuclei. A recently identified feature in their optical light curves is the presence of early bump-like structures (precursors) that appear before the onset of the main flare or during its rise. Our aim was to build and study the first sample of precursor TDEs in order to improve our understanding of these features, which could be key to revealing the origin of the optical emission in TDEs. We compiled all known precursor TDEs from the literature, searched for additional candidates, and analyzed them as a sample. We find that precursor TDEs predominantly fall within the subclasses of repeating TDEs, fast TDEs, and TDEs in active galactic nuclei (AGNs). We reveal a positive correlation between the occurrence time of the precursors relative to the main peak and the central black hole mass. We suggest that the precursors appear due to interactions between the incoming stellar debris and the disk or leftover material from an earlier disruption (repeating and fast TDEs) or a stable pre-existing disk (TDEs in AGNs). Precursors are therefore potentially key signatures of repeating partial TDEs in previously quiescent galaxies.