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Triple radio flares from tidal disruption events: jet-wind collisions and the discovery of a third radio flare from AT2020vwl

This paper presents a theoretical framework predicting a third radio flare in tidal disruption events caused by the collision of a fast jet with a slower wind, and confirms this prediction through the discovery of such a flare in the TDE AT2020vwl while forecasting similar future events for ASASSN-15oi and AT2024tvd.

Original authors: Andrew Mummery, Adelle Goodwin, Colin Christy, Kate Alexander, Noah Franz

Published 2026-07-20

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CC BY 4.0

Original authors: Andrew Mummery, Adelle Goodwin, Colin Christy, Kate Alexander, Noah Franz

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

Imagine the center of a galaxy as a cosmic playground dominated by a supermassive black hole, a gravitational giant so heavy it can swallow stars whole. When a star wanders too close, the black hole's tidal forces rip it apart in a spectacular event called a Tidal Disruption Event (TDE). Think of it like a cosmic spaghetti-fication, where the star is stretched into a long stream of gas before being devoured. As this gas spirals inward, it forms a hot, swirling disk and shoots out powerful blasts of energy. Astronomers watch these events across the universe, but the radio signals they receive are like a complex radio drama: sometimes the signal pops up immediately, sometimes it arrives years later, and sometimes it seems to have multiple acts. The big question is: what is the script? Are these blasts coming from one engine, or is the black hole switching between different modes of operation? Understanding this helps us figure out how black holes eat, how they launch energy, and how they shape the galaxies they live in.

In this paper, the authors propose a specific script for a three-act radio drama and then find the first real evidence that it's playing out. They suggest that some black holes don't just have one type of outflow; they have two distinct ones that happen at different times. First, when the black hole is feasting on a huge amount of gas, it blows a slow, thick "wind" that pushes against the surrounding space, creating the first radio flare. Later, as the black hole's meal slows down, it switches gears and launches a much faster, focused "jet." Because the jet is faster, it eventually catches up to the slower wind it left behind. The authors predict that when this fast jet slams into the dense shell of the old wind, it should create a third, distinct radio flare—a cosmic "crash" that lights up the radio sky again.

The team tested this idea by looking at a specific event called AT2020vwl. Using a computer model, they tracked the speed and position of the first two flares (the wind and the jet) to predict exactly when the jet would catch the wind. Their math said the collision should happen around 1,138 days after the star was destroyed. Then, they went back to their telescopes and kept watching. And there it was: a third radio flare appeared right on schedule, starting around day 1,334. The timing matched their prediction, and the radio signal behaved exactly as expected for a fast object hitting a slow, dense wall. This discovery is the first time a third flare in a TDE has been predicted in advance based on the physics of the first two.

The authors also looked at two other TDEs to see if the script holds up elsewhere. For one, ASASSN-15oi, the math suggests the jet will catch the wind, but the collision will be a gentle bump rather than a crash, so the third flare might be too faint to see. For another, AT2024tvd, the jet is moving so fast that a collision is predicted to happen very soon, potentially any day now. The paper also rules out several other ideas that could explain these flares, such as the outflow hitting a random cloud of gas or the black hole launching a completely new, third blast. The evidence points strongly to the "wind-jet collision" being the real culprit. It's a bit like watching a slow-moving delivery truck (the wind) leave a city, followed by a speeding sports car (the jet); if you know their speeds, you can predict exactly where and when the sports car will catch up, and the resulting bump in the road is the third flare. This discovery gives astronomers a powerful new tool to understand the hidden mechanics of black holes.

Technical Summary: Triple Radio Flares from Tidal Disruption Events

Problem and Motivation
Radio flares from tidal disruption events (TDEs) exhibit a diversity of timing, appearing either promptly (near the optical peak) or with significant delays (hundreds to thousands of days later). A recent framework (Goodwin & Mummery 2026b) proposes that this diversity arises from two physically distinct outflows: a prompt, super-Eddington wind launched during high accretion rates ( $\dot{m} \gtrsim 1$ ), and a delayed, faster jet launched during a disk state transition at low accretion rates ( $\dot{m} \lesssim 0.03$ ). A unique, unavoidable prediction of this dual-outflow framework is that because jets generally expand faster than winds, the delayed jet must eventually overtake the wind-blown shell. This collision, occurring at a time determined purely by the kinematics of the first two flares, should produce a third radio flare due to a sudden increase in the density encountered by the jet, altering its synchrotron self-absorption frequency ( $\nu_a$ ) and flux.

Methodology
The authors develop a forward model to quantify the dynamics of relativistic outflows with continuous energy injection. The model treats each flare as a mass-loaded outflow shocking its ambient medium and radiating synchrotron emission.

Dynamics: The model solves coupled integro-differential equations for the shock front radius $R(t)$ and Lorentz factor $\Gamma(t)$ , accounting for mass loading from the circumnuclear medium (CNM) and continuous mass injection from the central engine. The wind propagates into a pristine Bondi density profile, while the delayed jet propagates through the gas swept up and re-arranged by the wind.
Collision Physics: The collision time ( $t_{coll}$ ) is defined kinematically when $R_{wind}(t) = R_{jet}(t)$ . The authors derive the observable consequences of this collision, specifically focusing on the jump in the self-absorption frequency $\nu_a$ . They show that when the jet enters the dense, shocked wind shell, $\nu_a$ increases, leading to a re-brightening of the source in optically thin bands. The magnitude of this enhancement depends on the density contrast, relative velocity, and shell compression.
Inference: The model is fitted to multi-band radio light curves of three TDEs: AT2020vwl, ASASSN-15oi, and AT2024tvd. Using an affine-invariant ensemble sampler (emcee), the authors infer posterior distributions for outflow parameters (injection velocity, mass, density profiles) and predict collision times.

Key Results

Discovery of a Third Flare in AT2020vwl:
- Forward modeling of the first two flares of AT2020vwl predicted a collision time of $t_{coll} = 1138^{+275}_{-126}$ days post-optical peak.
- New long-term monitoring observations (VLA and GMRT) between 1334 and 1718 days revealed a distinct third radio flare.
- The onset of this flare coincides with the predicted collision window.
- Spectral analysis shows a clear increase in the synchrotron self-absorption break ( $\nu_a$ ) during the third flare, with a ratio $\nu_{a, coll}/\nu_{a, jet} = 1.7 \pm 0.2$ . This increase is consistent with the jet encountering the denser wind shell.
- Crucially, the inferred shock radius remains continuous across the transition from the second to the third flare, arguing against the launch of a third distinct outflow and supporting the collision scenario.
Predictions for Other Sources:
- ASASSN-15oi: Modeling predicts a collision at $t_{coll} \approx 2187$ days. However, the jet only marginally overtakes the wind, resulting in a low relative velocity and insufficient density contrast to produce a strong flare. The authors predict at most a weak change or flattening in the light curve, consistent with current sparse data showing no clear third flare.
- AT2024tvd: This source exhibits two flares separated by only $\sim 100$ days. The model predicts an imminent collision at $t_{coll} \approx 216^{+15}_{-13}$ days. The jet is significantly faster, and the density contrast is high, predicting a prominent third flare. A slight flux jump was observed at the final epoch ( $t=284$ days), potentially signaling the onset of this event.

Significance and Claims
The paper claims to present the first TDE radio flare predicted in advance based solely on the dynamics of earlier flares. The detection of the third flare in AT2020vwl, coincident with the kinematic prediction and exhibiting the specific spectral signature (increase in $\nu_a$ without a discontinuity in shock radius) expected from a wind-jet collision, provides strong observational support for the dual-outflow framework.

The authors argue that this mechanism explains the diversity of TDE radio light curves and offers a new probe of the circumnuclear density structure. They explicitly rule out alternative explanations for the third flare in AT2020vwl, such as:

Bondi radius crossing: The density change in a Bondi sphere is too gradual to produce a sharp flare.
Cloud collisions: While possible, the coincidence of cloud encounters with specific disk state transitions and accretion rates is considered fine-tuned.
Third distinct outflow: The continuous evolution of the shock radius and the lack of a physical mechanism for a third disk-driven outflow at low accretion rates make this unlikely.
Repeating partial TDEs: Ruled out by the single optical/UV flare observed in AT2020vwl.

The study concludes that third radio flares are a generic, predictable feature of TDEs that launch both a prompt wind and a delayed jet, serving as an independent test of the wind-plus-jet picture. The authors note that systematic uncertainties in density profiles and injection histories affect the precise timing of predicted collisions (particularly for AT2024tvd), but the qualitative prediction of a third event remains robust.

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