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Explaining the X-ray Precursor, Ultra-long Prompt Emission, and Week-long Decay of GRB250702B with a Jetted Micro-TDE

The paper proposes that the multi-phase evolution of the ultra-long gamma-ray burst GRB250702B, including its X-ray precursor, week-long prompt emission, and extended decay, is naturally explained by a jetted micro-tidal disruption event where a spinning stellar-mass black hole disrupts a Sun-like star, forming a stable polar funnel that allows a relativistic jet to propagate and produce the observed emission phases.

Original authors: Fulya Kıroğlu, Taeho Ryu, Alexander Tchekhovskoy, Kyle Kremer, Daichi Tsuna, Brian D. Metzger

Published 2026-08-12

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Fulya Kıroğlu, Taeho Ryu, Alexander Tchekhovskoy, Kyle Kremer, Daichi Tsuna, Brian D. Metzger

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 cosmic stage where the most dramatic shows are put on by stars and black holes. Sometimes, a massive star runs out of fuel and collapses, or two dense objects crash together, unleashing a blinding flash of energy called a Gamma-Ray Burst (GRB). These are the universe's brightest explosions, visible across billions of light-years. While we know the usual suspects for these shows—collapsing giant stars or smashing neutron stars—astronomers have recently spotted a very strange, ultra-long version of the show that doesn't fit the old script. It lasts for hours instead of seconds, starts with a quiet whisper of X-rays, and fades away over weeks. To understand this mystery, we need to know a few things: a "black hole" is a region of space so heavy that nothing, not even light, can escape its grip; a "tidal disruption event" is what happens when a star gets too close to a black hole and gets stretched into spaghetti-like strands; and a "jet" is a super-fast beam of energy shot out from the black hole's poles. The big question is: what kind of cosmic engine can keep a jet running for so long without running out of fuel?

This paper investigates a specific, record-breaking event called GRB 250702B, which lasted for seven hours and was preceded by a soft X-ray flare a day earlier. The authors, a team of astrophysicists, propose that this wasn't a dying giant star, but rather a "micro-tidal disruption event." Picture a small, stellar-mass black hole (about 10 times the mass of our Sun) eating a Sun-like star. Instead of a quick bite, the star gets torn apart into a swirling disk of debris that feeds the black hole for days. Using powerful computer simulations, the team shows how this messy meal creates a perfect tunnel through the debris, allowing a laser-like jet to escape and light up the sky. They found that if the black hole is spinning very fast, it can launch a jet powerful enough to explain the explosion, and the shape of the debris naturally keeps the jet stable as it shoots out.

The story of GRB 250702B is like a three-act play, and this paper explains how a single event—a black hole devouring a star—produces all three acts. The first act is the "X-ray precursor," a soft flare that happens about a day before the main show. The authors suggest this is caused by the very first bits of the star, the "stream" of stellar spaghetti, falling directly into the black hole before the main disk even forms. It's like the appetizer arriving before the main course is plated.

The second act is the main event: the ultra-long gamma-ray burst. In their simulations, the authors show that as the stellar debris settles into a disk, it naturally clears out a low-density "funnel" or tunnel along the poles (the top and bottom of the disk). This is crucial because if the jet had to push through thick, heavy gas, it would get stuck. But with this empty tunnel, the jet can zoom through. The team calculates that for the jet to be powerful enough to create the observed explosion (with a luminosity of about $10^{51}$ erg s $^{-1}$ ), the black hole must be spinning incredibly fast (a spin parameter of about 0.9). They also found that the jet must be extremely narrow, like a laser pointer with a beam width of less than 1 degree, to concentrate that energy into a blinding flash.

The third act is the long, slow fade. After the initial burst, the X-ray light doesn't just stop; it declines over weeks. The authors explain this using a model where the disk of debris is constantly losing mass to powerful winds. As the disk spreads out and gets thinner, the fuel supply to the black hole drops, causing the jet's power to fade. Interestingly, the light curve (the graph of brightness over time) gets steeper at first because the jet itself is "widening" like a garden hose nozzle being opened up, spreading the light over a larger area. This combination of a fading engine and a widening beam perfectly matches the sharp drop in brightness observed in the first few days. Later, once the jet stops widening, the light fades at a steady, slower pace, matching the weeks-long tail seen by telescopes.

The paper explicitly rules out the idea that this was a standard collapsing star (a "collapsar") because those models struggle to explain the day-long X-ray precursor and the lack of a supernova explosion. They also argue against the idea that the black hole was a supermassive one in the center of a galaxy, because the event happened far away from the galaxy's center. Instead, their simulations of a 10 solar-mass black hole tearing apart a 1 solar-mass star provide a consistent picture for all the observations.

One of the most exciting findings is that the jet doesn't just survive; it stays stable. In physics, powerful jets can sometimes twist and snap apart due to a "kink instability," like a garden hose that kinks and stops the water. The authors used their simulation data to check if the jet would survive the journey through the debris. They found that the specific shape of the debris tunnel (where the density drops off in a very specific way) acts like a stabilizer, keeping the jet straight and strong all the way out. This suggests that micro-TDEs are not just a theoretical possibility but a physically robust engine for these ultra-long cosmic fireworks.

In summary, this paper suggests that GRB 250702B was likely a "micro-TDE" where a fast-spinning, stellar-mass black hole ate a Sun-like star. The simulations show that this event naturally creates a clear path for a jet to escape, explains the three distinct phases of the explosion, and keeps the jet stable enough to be seen from Earth. While the authors are confident in their simulations and the physical logic, they note that future, even more detailed computer models will be needed to confirm exactly how the magnetic fields and the spinning black hole interact to produce such a spectacular show. For now, this model offers a compelling new chapter in our understanding of how the universe's most energetic explosions can happen.

Technical Summary: Explaining the X-ray Precursor, Ultra-long Prompt Emission, and Week-long Decay of GRB 250702B with a Jetted Micro-TDE

Problem Statement
GRB 250702B represents the longest detected gamma-ray burst (GRB) on record, exhibiting a complex multi-phase light curve: a soft X-ray precursor approximately one day prior to the main event, followed by $\sim$ 7 hours of prompt $\gamma$ -ray emission, and a weeks-long fading X-ray tail. The phenomenology of this event—specifically the lack of a supernova counterpart, the large offset from the host galaxy nucleus (5.7 kpc), and the extended duration—presents a tension with standard core-collapse (collapsar) models. While several progenitor channels have been proposed for ultra-long GRBs (ULGRBs), including the collapse of blue supergiants, Population III stars, or tidal disruption events (TDEs) by intermediate-mass black holes, a unified physical model explaining all three phases of GRB 250702B has remained elusive. A critical unresolved question is whether a micro-TDE (the tidal disruption of a main-sequence star by a stellar-mass black hole) can launch a relativistic jet capable of escaping the dense debris envelope without being disrupted by magnetohydrodynamic instabilities.

Methodology
The authors employ a multi-stage approach combining 3D hydrodynamic simulations with semi-analytic modeling:

Hydrodynamic Simulations: Using the moving-mesh code arepo, the authors simulate the tidal disruption of a $1 M_\odot$ Sun-like star by a $10 M_\odot$ black hole. They model two encounter geometries: a grazing encounter ( $\beta = r_T/r_p = 1.4$ ) and a deep encounter ( $\beta = 5$ ). The simulations resolve the debris envelope formation up to $\sim$ 1 day post-disruption, employing adaptive mesh refinement to capture the disk structure near the black hole.
Disk Evolution Model: Since arepo cannot resolve the long-term viscous evolution, the authors extract the fallback rate ( $\dot{M}_{fb}$ ) from the simulations and input it into a semi-analytic model. This model incorporates wind-driven mass loss (parameterized by a slope $s$ ) and viscous accretion to track the disk mass ( $M_d$ ) and radius ( $r_d$ ) evolution over weeks.
Jet Stability Analysis: The authors apply an analytic jet-stability framework to the density profiles derived from the simulations. They evaluate the kink-instability stability parameter ( $\Lambda$ ) for jets propagating through the polar funnel of the debris envelope, considering various jet powers ( $L_{jet}$ ), Lorentz factors ( $\gamma_j$ ), and opening angles ( $\theta_j$ ).
Application to GRB 250702B: The derived accretion rates and jet powers are used to model the isotropic-equivalent luminosities ( $L_{iso}$ ) for the X-ray precursor, prompt $\gamma$ -ray, and late-time X-ray phases, comparing the results against observational data from Einstein Probe, Fermi, Swift, and Chandra.

Key Contributions and Results

Debris Envelope Geometry: The simulations reveal that within $\sim$ 1 day, the disrupted debris forms a quasi-steady envelope with a low-density polar funnel ( $\rho \propto r^{-2}$ ) and a half-opening angle of $\approx 15^\circ$ . This geometry is robust across both grazing and deep encounter scenarios.
Jet Stability: The $r^{-2}$ polar density profile is identified as critical. It provides a confining pressure that declines steadily, keeping the jet below the kink-instability threshold ( $\Lambda > 2$ ) for jet powers $L_{jet} \gtrsim 10^{47}$ erg s $^{-1}$ . This confirms that jets can propagate stably through the micro-TDE debris without stalling, provided the black hole is rapidly spinning ( $a_\bullet \sim 0.9$ ) to generate sufficient power via the Blandford–Znajek mechanism.
Unified Model for GRB 250702B: The authors propose a single physical engine (a jetted micro-TDE) that naturally reproduces all three observed phases:
- X-ray Precursor ( $\sim$ 1 day prior): Attributed to pre-disk, stream-fed accretion where a small fraction ( $\lesssim 1\%$ ) of debris falls directly onto the black hole before the disk fully forms. This yields a lower luminosity ( $L_{X,iso} \sim 10^{46}$ erg s $^{-1}$ ) and is not beamed as tightly as the prompt emission.
- Prompt $\gamma$ -Ray Emission: Powered by a tightly beamed jet ( $\theta_b \lesssim 1^\circ$ ) launched by a rapidly spinning black hole ( $a_\bullet \sim 0.9$ ) once the polar funnel is cleared. The model reproduces the observed isotropic luminosity ( $L_{\gamma,iso} \sim 10^{51}$ erg s $^{-1}$ ) with a jet power of $L_{jet} \approx 3 \times 10^{47}$ erg s $^{-1}$ .
- Weeks-long X-ray Decay: The late-time light curve is explained by wind-modified viscous accretion ( $\dot{M}_{acc} \propto t^{-2}$ $\dot{M}_{a cc} \propto t^{- 2}$ ) combined with jet widening.
  - During the intermediate EP-WXT phase, the jet opening angle widens linearly with time ( $\theta_b \propto t$ ), steepening the observed decay to $L_{X,iso} \propto t^{-4}$ (observed $\alpha \approx 3.8$ ).
  - In the late EP-FXT phase, the jet angle saturates, and the light curve tracks the intrinsic engine decay ( $L_{jet} \propto t^{-2}$ ), matching the observed week-long tail. The duration is extended by the viscous spreading of the disk ( $r_d \propto t^{2/3}$ ).

Significance
The paper establishes jetted micro-TDEs as a physically motivated engine for ULGRBs. By demonstrating that the debris geometry naturally creates a stable escape channel for relativistic jets, the authors resolve the primary theoretical hurdle preventing micro-TDEs from being viable GRB progenitors. The model successfully accounts for the unique temporal structure of GRB 250702B—including the precursor, the ultra-long prompt duration, and the extended decay—without invoking multiple engines, repeated partial disruptions, or external shock afterglows. The work suggests that the specific combination of a rapidly spinning stellar-mass black hole and a Sun-like star in a micro-TDE scenario can produce the extreme energetics and timescales observed in the longest GRBs.

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