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One year of broadband radio monitoring of the enigmatic transient GRB 250702B reveals the evolution of the relativistic jet

This paper presents a one-year broadband radio monitoring campaign of the transient GRB 250702B, revealing a smoothly evolving synchrotron spectrum and interstellar scintillation that suggest the event is powered by either a narrow, relativistic jet from a stellar-mass black hole merger or a wide-angle jet from a tidal disruption event involving an intermediate-mass or stellar-mass black hole, while ruling out a supermassive blackhole origin.

Original authors: A. J. Goodwin, James C. A. Miller-Jones, Itai Sfaradi, Andrew Mummery, Raffaella Margutti, Tanmoy Laskar, K. D. Alexander, Yuhan Yao, Arvind Balasubramanian, G. C. Anupama, Edo Berger, Varun Bhalerao

Published 2026-08-05

📖 7 min read🧠 Deep dive

CC BY 4.0

Original authors: A. J. Goodwin, James C. A. Miller-Jones, Itai Sfaradi, Andrew Mummery, Raffaella Margutti, Tanmoy Laskar, K. D. Alexander, Yuhan Yao, Arvind Balasubramanian, G. C. Anupama, Edo Berger, Varun Bhalerao, Yvette Cendes, Ryan Chornock, C. T. Christy, D. Eappachen, Tarraneh Eftekhari, Miguel Pérez-Torres, Enrico Ramirez-Ruiz, D. K. Sahu, Sjoert van Velzen

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 universe as a vast, dark ocean, and scattered throughout it are cosmic lighthouses. Most of these lighthouses flash briefly—a quick burst of light from a dying star or a colliding pair of neutron stars—before fading into the dark. But every now and then, something strange happens. A lighthouse doesn't just flash; it screams, roars, and keeps screaming for hours, days, or even weeks, blasting energy across the entire electromagnetic spectrum. Astronomers call these events Gamma-Ray Bursts (GRBs). They are the most energetic explosions since the Big Bang, capable of outshining entire galaxies for a split second.

To understand what's happening during these explosions, scientists act like cosmic detectives. They don't just look at the bright flash; they watch the "afterglow," the fading echo of light that lingers long after the initial boom. By listening to this echo in radio waves (the same kind of waves that carry music to your car), they can figure out how fast the explosion is moving, how much energy it has, and what kind of "wind" or dust cloud it is crashing into. The big question is: what kind of engine is driving these explosions? Is it a massive star collapsing in on itself, or is it a hungry black hole tearing a star apart? Sometimes, the clues are so weird that they don't fit into any of the known categories, forcing scientists to invent new stories about how the universe works.

The Mystery of the Long-Lasting Roar

In July 2025, a cosmic event named GRB 250702B lit up the sky, and it was a total oddball. While most gamma-ray bursts last for a few seconds or maybe a few minutes, this one kept screaming for over 25,000 seconds—more than seven hours! It was the longest gamma-ray burst ever recorded. But the real mystery wasn't just how long it lasted; it was what happened after the initial scream.

A team of astronomers decided to play the role of cosmic eavesdroppers. They pointed some of the world's most powerful radio telescopes (including the Very Large Array in New Mexico and the Atacama Large Millimeter Array in Chile) at the spot where the explosion happened. They didn't just look once; they kept watching for a full year, from 6 days after the blast all the way to 356 days later. They were listening to the "radio afterglow," the sound of the explosion's shockwave plowing through space.

What They Found: A Shifting Wave

The team found that the radio signal didn't just fade away quietly. Instead, it behaved like a complex, shifting wave. At high frequencies (like the high-pitched notes of a violin), the signal died down quickly. But at lower frequencies (like the deep rumble of a bass drum), the signal hung around, getting brighter for a while before slowly fading.

This pattern told the scientists that the explosion was pushing a massive, expanding shell of gas into a surrounding environment that wasn't empty. It was like a speedboat driving through water that got thicker and thicker the further out you went. The data suggested the explosion was moving through a "stratified" medium, meaning the density of the gas dropped off in a specific way, similar to the wind blowing off a massive star or the gas swirling around a black hole.

The Size of the Blast: A Cosmic Shimmer

One of the coolest things the team noticed was that the low-frequency radio signals were flickering wildly, like a candle flame in a drafty room. They figured out this wasn't the explosion itself changing; it was the Earth's atmosphere (specifically, the ionized gas in our galaxy) acting like a wobbly lens, distorting the view of the distant object. This effect is called "interstellar scintillation."

By measuring how much the signal shimmered, they could put a size limit on the explosion. They calculated that the blast wave was roughly the size of a giant bubble, with a radius between $1.2 \times 10^{16}$ and $5 \times 10^{17}$ centimeters. To put that in perspective, that's a few hundred times the distance from the Earth to the Sun, but tiny compared to the size of a galaxy.

Two Possible Engines: The Narrow Jet vs. The Wide Fan

The big question remained: What kind of engine created this monster? The data pointed to two very different possibilities, like a car that could be either a super-fast Formula 1 racer or a slow-moving, wide delivery truck.

The Narrow Jet (The F1 Racer): The explosion could be a super-tight beam of energy, shooting out in a narrow cone (less than 2 degrees wide) at incredible speeds. If this were the case, the total energy would be huge, but because the beam is so narrow, we only see it if we are looking right down the barrel. This fits the profile of a "collapsar," where a massive star collapses into a black hole.
The Wide Fan (The Delivery Truck): Alternatively, the explosion could be a much wider, slower-moving blast (spreading out over 15 degrees or more) that isn't moving as fast. This would require a different kind of engine, perhaps a black hole tearing apart a star in a "Tidal Disruption Event" (TDE).

The team couldn't say for sure which one it was yet. However, they did rule out some ideas. They are pretty confident it's not a standard, massive star collapsing in the usual way, because those engines usually run out of fuel too quickly to last for seven hours. They also ruled out the idea that a super-massive black hole (the kind sitting in the center of a galaxy) was the culprit, because the timing of the explosion didn't match the slow-motion physics of those giants.

The Best Guess: A Star-Eating Black Hole

So, what is the most likely story? The authors suggest the explosion was likely caused by a black hole (either a "stellar-mass" one, similar in weight to our Sun, or a medium-sized "intermediate-mass" one) tearing apart a star.

If it was a black hole eating a white dwarf (a dead, dense star), the engine would run for a shorter time, maybe a few hundred days. If it was eating a normal, living star, the engine could run for years. The fact that the radio signal is fading smoothly without needing extra "fuel injections" suggests the engine is running on a steady, powerful diet of star-stuff.

The team is keeping a close eye on the signal. If the radio waves suddenly stop within the next year, it would be a huge clue. It would mean the black hole finished eating the star and the engine shut off, which would strongly point to the "white dwarf" scenario. If the signal keeps going for years, it might mean a normal star is being slowly devoured.

Why It Matters

GRB 250702B is a cosmic puzzle piece that doesn't quite fit the picture we have so far. It has the long duration of a TDE but the high-energy punch of a GRB. By studying how its radio waves change over time, scientists are learning that the universe has more ways to make a mess than we thought. Whether it's a narrow, super-fast jet or a wide, slow-moving fan, this event proves that when black holes get hungry, they can put on a show that lasts for weeks, challenging our understanding of how stars die and how black holes wake up.

Technical Summary: One year of broadband radio monitoring of the enigmatic transient GRB 250702B reveals the evolution of the relativistic jet

Problem Statement
GRB 250702B is an unprecedented extragalactic transient characterized by a prompt gamma-ray duration exceeding 25,000 seconds, a pre-burst X-ray detection, and an off-nuclear location within a massive, dusty host galaxy at $z=1.036$ . These properties challenge standard collapsar models, which cannot sustain accretion for such durations, and complicate the distinction between ultra-long GRBs (ulGRBs) and relativistic jetted tidal disruption events (TDEs). While initial multiwavelength modeling suggested a narrow jet, limited radio data left the nature of the outflow, the ambient environment, and the progenitor system (e.g., helium star merger vs. various TDE channels) largely unconstrained. The primary problem addressed is determining the physical properties of the relativistic outflow and identifying the progenitor system through extensive, broadband radio monitoring.

Methodology
The authors conducted a comprehensive radio monitoring campaign of GRB 250702B spanning 6 to 356 days (observer frame) post-discovery. Observations were obtained across a frequency range of 0.65–233 GHz using the Karl G. Jansky Very Large Array (VLA), the Atacama Large Millimeter/submillimeter Array (ALMA), and the upgraded Giant Metrewave Radio Telescope (uGMRT), supplemented by archival MeerKAT data.

The analysis involved:

Light Curve Modeling: Fitting individual frequency light curves with a smoothly broken power-law to identify break times and decay indices.
Interstellar Scintillation (ISS) Analysis: Analyzing low-frequency variability ( $\leq 3$ GHz) to constrain the source size and blastwave image radius using the Walker (1998) formalism.
Spectral Modeling: Fitting broadband spectra (0.65–233 GHz) with synchrotron emission models (Granot & Sari 2002) to track the evolution of critical frequencies ( $\nu_m$ , $\nu_a$ , $\nu_c$ ) and the electron power-law index ( $p$ ). X-ray data were extrapolated and included to test for cooling breaks.
Equipartition Analysis: Estimating physical parameters (radius, energy, magnetic field, ambient density, Lorentz factor) under the assumption of energy equipartition between electrons and magnetic fields, considering various jet geometries (narrow vs. wide-angle).
Progenitor Assessment: Comparing the derived timescales, energetics, and environmental density profiles against theoretical models for stellar collapse (helium star mergers) and TDEs involving stellar-mass, intermediate-mass (IMBH), and supermassive black holes (SMBH).

Key Results

Synchrotron Evolution: The radio emission exhibits a smoothly evolving, peaked synchrotron spectrum consistent with an adiabatic shock expanding into a stratified ambient medium ( $n_e \propto R^{-k}$ with $k \approx 1.5-2$ ). The temporal evolution of flux density and critical frequencies suggests the shock is likely a forward shock, potentially with a minor reverse shock component.
Source Size Constraints: Significant variability in the 1.25 and 3 GHz light curves is interpreted as interstellar scintillation. This places an approximate bound on the blastwave image size of $1.2 \times 10^{16} \lesssim R_{\perp} \lesssim 5 \times 10^{17}$ cm at early times.
Jet Geometry and Lorentz Factor: The absence of a jet break in the radio (6–356 d) or X-ray ( $>0.5$ $> 0.5$ d) light curves implies two viable scenarios:
1. A narrow jet ( $\theta_j \lesssim 2^\circ$ ) with a high initial Lorentz factor ( $\Gamma_0 \gtrsim 80$ ) that has undergone significant deceleration to $\Gamma \lesssim 10$ by day 6.
2. A wide-angle jet ( $\theta_j \gtrsim 15^\circ$ ) with a low Lorentz factor ( $\Gamma \lesssim 10$ ).
Ambient Environment: The density profile is consistent with a stellar wind or Bondi accretion profile ( $k \approx 1.5-2$ ), ruling out a constant density interstellar medium.
Energetics:
- In the narrow jet scenario, the beaming-corrected kinetic energy is $E_K \sim 10^{51}$ erg, consistent with the distribution of long GRBs.
- In the wide-angle scenario, $E_K \sim 10^{50}$ erg, consistent with the lower end of relativistic TDE distributions.
Progenitor Constraints:
- SMBH TDEs are disfavored due to incompatibility with the observed prompt timescales and the off-nuclear location.
- Stellar-mass black hole (sBH) mergers or TDEs involving sBHs or IMBHs disrupting main-sequence stars or white dwarfs remain viable.
- The short variability timescales favor lower-mass engines (sBH or IMBH). The specific timing of the X-ray precursor ( $\sim 1$ day) favors sBH-MS or IMBH-MS channels over the faster IMBH-WD channel, though the latter is not strictly ruled out if additional physics (e.g., stream self-intersection) is invoked.
- The detection of a jet shut-off in the future would favor a WD-IMBH TDE due to the shorter theoretical duration of super-Eddington accretion compared to main-sequence channels.

Significance and Claims
The paper claims that GRB 250702B represents a unique class of transient that bridges the gap between standard GRBs and relativistic TDEs. The authors assert that their radio monitoring provides the first robust constraints on the jet geometry and environmental density of this event, moving beyond the initial narrow-jet hypothesis.

Key contributions include:

Validation of Progenitor Scenarios: The work demonstrates that both a helium star merger (collapsar variant) and specific TDE channels (sBH or IMBH disrupting a star) are physically viable, provided the jet geometry and Lorentz factor align with the derived constraints.
Differentiation of TDE Channels: By analyzing the timescales of fallback, viscous accretion, and super-Eddington supply, the authors show that the observed properties are incompatible with a classical SMBH TDE but consistent with lower-mass black hole disruptions.
Future Diagnostic: The paper posits that the detection of a jet shut-off within the next year would serve as a critical discriminant, specifically favoring a WD-IMBH TDE scenario due to the predicted duration of the accretion phase.

The authors conclude that while the event does not fit cleanly into existing categories, the combination of radio afterglow evolution, environmental stratification, and progenitor timescales offers a pathway to understanding the diversity of relativistic outflows in the universe. They emphasize that future multiwavelength modeling is required to fully disentangle the contributions of forward and reverse shocks and to definitively constrain the jet opening angle.

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