Precursors in tidal disruption events: repeating, fast, and AGN-hosted TDEs

TL;DR

The paper addresses the origin of early precursor bumps in optical TDE light curves. It constructs the first uniform sample of precursor TDEs across repeating, fast, and AGN-hosted events by compiling literature cases and reanalyzing photometry with forced observations, revealing timing correlations with black hole mass. A key finding is a positive relation between precursor occurrence time and $M_{BH}$, described by $log(M_{BH}/M_\odot) = (1.03 \pm 0.26) log(t_{occ}/days) + 5.04$, supporting a debris-disk interaction origin where returning stellar debris intersects disk material at larger radii for more massive black holes. The results suggest precursors are key signatures of repeating partial TDEs and advocate for early multiwavelength follow-up to constrain the physical mechanism and monitor future flares in known systems.

Abstract

Context. 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. Aims. We aim 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. Methods. We compiled all known precursor TDEs from the literature, searched for additional candidates, and analyzed them as a sample. Results. We find that precursor TDEs predominantly fall within the repeating TDE, fast TDE, and TDE in active galactic nucleus (AGN) subclasses. We reveal a positive correlation between the occurrence time of the precursors relative to the main peak and the central black hole mass. Conclusions. 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.

Figures (5)

• Figure 1: A positive trend between the earlier occurrence time of the precursors and the mass of the central SMBH. We define the occurrence time when the optical emission reached its highest value during the precursor flare and the x-axis uncertainties represent the duration of the precursor. The black hole mass estimates are explained in the body text in detail. We excluded AT2022fpx from the fit due to lack of observations around the precursor.
• Figure 2: Optical light curves of our precursor sample. We show the differential (i.e., host-subtracted) light curves, with the only exception of AT2023uqm, whose precursor flare mainly exhibits negative fluxes (i.e., fainter than the baseline emission); in this case, we show the aperture photometry. The precursor features are highlighted with gold areas. For ASASSN-14ko — in which case the re-brightening during decline was studied earlier in detail — we also highlight this phase with an orange shaded area. Days are always in rest-frame. The light curves were binned only for intranight observations to improve the signal-to-noise ratio, unless stated otherwise in the figure annotations.
• Figure 3: Long-term optical light curve of the spectroscopically confirmed repeating pTDE AT2022dbl. We plotted ZTF aperture magnitudes in order to be consistent with CRTS and Gaia photometries which are not corrected for baseline. No enhanced infrared emission was detected by WISE after the first optical flare while no data is available after the second.
• Figure 4: Long-term optical light curve of the repeating TDE AT2019azh presented in aperture magnitudes. The second flare appeared $14$ years after the first one detected by CRTS. Two individual TDE scenarios and a repeating pTDE scenario are discussed in more detail in the main text. The second flare was accompanied by an IR dust echo flare whilst no data is available after the first flare for comparison.
• Figure 5: Long-term optical light curve of the repeating TDE AT2024pvu presented in aperture magnitudes. The second flare appeared $18$ years after the first one detected by CRTS. Two individual TDE scenarios and a repeating pTDE scenario are discussed in more detail in the main text. Unfortunately, no WISE observations were obtained after the second optical flare due to the satellite’s shutdown.