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

This paper presents the first systematic study of tidal disruption event precursors, revealing that they predominantly occur in repeating, fast, or AGN-hosted events and suggesting they arise from interactions between stellar debris and pre-existing or leftover disks.

Original authors: Patrik Milán Veres

Published 2026-03-25

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CC BY 4.0

Original authors: Patrik Milán Veres

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "Warning Shot": What TDE Precursors Are

Imagine a supermassive black hole as a giant, invisible vacuum cleaner sitting at the center of a galaxy. Usually, it's quiet. But occasionally, a star wanders too close. The black hole's gravity is so strong that it rips the star apart like a piece of taffy being pulled by two kids. This event is called a Tidal Disruption Event (TDE).

When the star gets shredded, about half of its remains crash into the black hole, creating a massive, glowing flash of light (a flare) that astronomers can see across the universe.

The Mystery:
For a long time, astronomers thought these flares were simple: a star gets eaten, and boom, a bright flash appears. But recently, they noticed something weird. Before the main "boom," some of these events show a little "bump" or a small flash of light. The authors of this paper call these "precursors."

Think of it like this: If the main flare is the sound of a car crash, the precursor is the sound of the car screeching its brakes just a split second before impact.

What Did the Scientists Do?

The author, Patrik Milán Veres, gathered a "mugshot" of every known TDE that had these precursors. He found 16 cases and asked: What do these have in common? Why do they happen?

He discovered that these "warning shots" only happen in three specific types of cosmic scenarios:

1. The "Repeat Offenders" (Repeating TDEs)

Sometimes, a star doesn't get eaten all at once. It gets a little bite taken out of it, survives, and swings back around the black hole to get bitten again next year.

The Analogy: Imagine a shark taking a bite out of a swimmer, letting them swim away, and then biting them again next week.
The Precursor: In these cases, the "precursor" is the splash and spray caused when the star swings back in and hits the leftover water (debris) from the previous bite. It's a collision between the star and the mess it left behind.

2. The "Speed Demons" (Fast TDEs)

Some TDEs fade away incredibly fast, much faster than the standard "textbook" prediction.

The Analogy: Most fireworks fade slowly, but these are like a sparkler that burns out in a split second.
The Discovery: The author found that these "speed demons" are actually just the second or third bite in a repeating event that we missed the first time. Because the star is already partially eaten, it falls apart faster. The "precursor" is the star hitting the debris from the first, unseen bite.

3. The "Party Hosts" (AGN-Hosted TDEs)

Some black holes are already active, surrounded by a swirling disk of gas and dust (an Active Galactic Nucleus or AGN).

The Analogy: Imagine a black hole is a busy dance floor full of people (gas and dust). When a star tries to crash the party, it doesn't just hit the DJ; it crashes into the crowd first.
The Precursor: The "bump" of light happens because the incoming star smashes into the pre-existing crowd (the disk) before it even gets close enough to be fully eaten by the black hole.

The Big Discovery: Size Matters

The most exciting part of the paper is a pattern the author found. He plotted the size of the black hole against when the precursor happened.

The Rule: The bigger the black hole, the earlier the precursor appears before the main crash.
The Analogy: Imagine throwing a pebble into a pond.
- If the pond is small (small black hole), the ripples (precursor) hit the edge quickly.
- If the pond is huge (massive black hole), the ripples have a long way to travel.
- Wait, that's backwards! Let's try a better one: Imagine a giant trampoline. If you drop a ball on a small trampoline, it bounces fast. If you drop it on a massive, deep trampoline, it takes longer to hit the bottom.
- Actually, the paper says: Larger black holes have larger "accretion disks" (the debris field). Because the disk is so huge, the star hits the outer edge of the debris field sooner in its journey than it would around a smaller black hole. So, the "warning shot" happens much earlier relative to the main crash.

Why Does This Matter?

This paper changes how we see the universe.

It's not just one big crash: Many TDEs we thought were single events are actually "repeating" events where a star is slowly being eaten over years.
The "Fast" ones are clues: The TDEs that fade quickly aren't weird; they are just the later stages of a star being eaten.
The "Precursor" is a key: If we see that little "bump" of light before the main flare, it tells us there is likely a disk of debris or a previous bite waiting for the star.

In short: The universe is giving us a "heads up" before the main event. By listening to these early bumps, astronomers can finally understand that black holes often don't just eat stars in one gulp; they nibble, spit out the leftovers, and come back for seconds.

1. Problem Statement

Tidal Disruption Events (TDEs) occur when a star is torn apart by the tidal forces of a supermassive black hole (SMBH). While the standard model predicts a single optical flare decaying as $t^{-5/3}$ , recent observations have identified a subset of TDEs exhibiting early, bump-like structures in their optical light curves prior to the main peak. These features, termed "precursors," challenge current understanding of TDE emission mechanisms.

The Gap: The physical origin of these precursors remains unclear. Proposed mechanisms include recombination radiation, stream-stream collisions, or shock breakouts.
The Context: Theoretical models suggest that partial TDEs (pTDEs), where a stellar core survives to produce recurrent flares, should be common, yet observationally confirmed cases are rare. Furthermore, TDEs occurring in Active Galactic Nuclei (AGN) are expected to be frequent but are difficult to distinguish from intrinsic AGN variability.
Objective: The paper aims to construct the first systematic sample of precursor TDEs to determine their physical origin and identify correlations with host galaxy properties (specifically SMBH mass) and TDE subclasses.

2. Methodology

The authors employed a multi-step observational and analytical approach:

Sample Compilation: They compiled a dataset of 16 transients from the literature and public photometric data (ZTF, ATLAS, ASAS-SN).
- Total Sample: 13 confirmed cases from literature + 3 newly identified candidates (AT2024pvu, AT2022dbl, AT2019aalc) based on visual inspection of light curves.
- Classification: The sample was categorized into three subclasses: Repeating TDEs (pTDEs), Fast TDEs (rapidly declining single flares), and TDE-AGNs (events in active nuclei).
Data Processing:
- Optical differential light curves were generated using forced photometry services.
- Galactic extinction corrections were applied using the Schlafly & Finkbeiner (2011) map.
- Precursor occurrence times were defined as the time of peak optical emission during the precursor phase relative to the main flare peak.
Black Hole Mass Estimation:
- SMBH masses ( $M_{BH}$ ) were collected from literature, prioritizing reverberation mapping.
- For missing data, masses were estimated via stellar velocity dispersion ( $\sigma_*$ ) or galaxy-mass scaling relations (Greene et al. 2020).
- The authors recalculated masses for three specific sources (AT2022dbl, AT2023lli, AT2024kmq) to ensure consistency with updated scaling relations.
Statistical Analysis: An orthogonal distance regression was performed on a log-log plot of precursor occurrence time ( $t_{occ}$ ) versus SMBH mass ( $M_{BH}$ ). Correlation strength was assessed using Pearson and Spearman coefficients.

3. Key Contributions

First Systematic Sample: This work presents the first comprehensive sample of TDEs with identified precursors, unifying previously disparate cases under a single analytical framework.
Subclass Correlation: The study establishes that precursors are not random anomalies but are strongly associated with specific TDE subclasses: Repeating TDEs, Fast TDEs, and TDE-AGNs.
Mass-Dependent Timing: The paper reveals a statistically significant positive correlation between the central black hole mass and the timing of the precursor relative to the main peak.
Unified Physical Model: The authors propose a unified mechanism explaining precursors across different subclasses, moving away from isolated explanations for each event type.

4. Key Results

Categorization:
- Repeating TDEs: 5 of the 6 investigated repeating TDEs (e.g., AT2022dbl, ASASSN-14ko) exhibit precursors. These are often interpreted as partial disruptions where the stellar core survives.
- Fast TDEs: Five single-flare TDEs (e.g., AT2020wey, AT2023lli) with the fastest known decline rates ( $p \approx -2.5$ to $-4.1$) also show precursors. Their decay rates ( $t^{-p}$ with $p \approx 2-5$ ) align better with pTDE simulations than standard full disruption models.
- TDE-AGNs: All identified TDE-AGN candidates in the sample (e.g., AT2021aeuk, AT2019aalc) display precursors.
Statistical Correlation:
- A log-log fit yields the relation: $\log(M_{BH}/M_\odot) = (1.03 \pm 0.26) \log(t_{occ}/\text{days}) + 5.04$ .
- Significance: The correlation is moderate-to-strong with a Pearson coefficient of 0.687 ( $p=0.006$ ) and a Spearman rank of 0.833 ( $p=0.0002$ ), corresponding to a 3.7 $\sigma$ significance.
- Interpretation: Higher mass black holes have larger Innermost Stable Circular Orbits (ISCO) and accretion disks. Consequently, returning stellar debris intersects the disk at larger radii and earlier times relative to the main peak, producing earlier/more pronounced precursors.
Physical Mechanism: The authors conclude that precursors arise from interactions between incoming stellar debris and a pre-existing disk (in AGNs) or leftover debris from a previous disruption (in repeating TDEs). The kinetic energy dissipated via shocks in these collisions produces the observed early optical bump.

5. Significance and Implications

Re-evaluating "Single" TDEs: The presence of precursors in "Fast TDEs" suggests that many events previously classified as full disruptions may actually be partial TDEs (pTDEs) or secondary flares from a previous partial disruption. This supports theoretical predictions (Bortolas et al. 2023) that pTDEs are more common than currently observed.
AGN Environment: The findings reinforce the idea that TDEs in AGNs are distinct due to the presence of stable accretion disks, which act as a target for the stellar stream, generating observable precursors.
Observational Strategy: The paper argues that early triggering of follow-up observations (including radio bands) is critical for new TDE discoveries. Detecting precursors can serve as a diagnostic tool to identify pTDEs in previously quiescent galaxies and distinguish TDE-AGNs from standard AGN variability.
Future Predictions: The study highlights upcoming flaring episodes (e.g., the third flare of AT2022dbl in early 2026) as opportunities to further test the repeating TDE hypothesis and the debris-disk interaction model.

In summary, this paper provides strong observational evidence that TDE precursors are a signature of debris-disk interactions, linking the timing of these features directly to the mass of the central black hole and suggesting that a significant fraction of observed TDEs are actually partial, repeating events.