🔭 astrophysics

Ultra-long Gamma-ray Bursts from Micro-Tidal Disruption Events: The Case of GRB 250702B

The paper proposes that the ultra-long gamma-ray burst GRB 250702B originates from a micro-tidal disruption event involving a stellar-mass black hole or neutron star disrupting a main-sequence star, offering a unified explanation for its unique multi-hour duration, X-ray pre-peak, and hard gamma-ray variability that standard progenitor models cannot account for.

Original authors: Paz Beniamini, Hagai B. Perets, Jonathan Granot

Published 2026-02-23

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Paz Beniamini, Hagai B. Perets, Jonathan Granot

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 Mystery: A "Super-Long" Firework Show

Imagine the universe is a dark stage, and occasionally, massive explosions happen that light it up for a split second. These are Gamma-Ray Bursts (GRBs). Usually, they are like camera flashes: incredibly bright, but they last only a fraction of a second (short bursts) or maybe a few minutes (long bursts).

But then, astronomers spotted a weirdo: GRB 250702B.

This wasn't a flash; it was a marathon. It blazed for over 4 hours. Even stranger, it started with a faint "whisper" of X-rays about 24 hours before the main explosion, and it happened far away from the center of its home galaxy, in a lonely spot.

The scientists in this paper (Beniamini, Perets, and Granot) are trying to solve the mystery: What kind of cosmic engine could last for hours, start with a warning shot, and happen in the middle of nowhere?

The Old Suspects (Why They Don't Fit)

The team first looked at the usual suspects for long explosions, but they didn't fit the clues:

The Blue Giant Star Collapse: Usually, when a massive star dies, it collapses into a black hole. But these usually come with a huge, bright supernova (a stellar funeral) that we didn't see. Also, they don't explain the 24-hour "warning shot" before the main event.
The Magnetar Engine: This is a super-magnetic neutron star spinning like a top. While it can power long bursts, it's hard to explain why it would start whispering a day early and then scream hours later.
The White Dwarf vs. Giant Black Hole: Imagine a tiny white dwarf star getting eaten by a super-massive black hole. This usually happens in the center of a galaxy. But our explosion happened in the suburbs. Plus, the timing was all wrong; it would have been over too fast.

The New Theory: The "Micro-Tidal Disruption Event" (The Cosmic Dance)

The authors propose a new culprit: a Micro-Tidal Disruption Event (μTDE).

The Analogy: The Hungry Dog and the Frisky Cat
Imagine a Black Hole (or a Neutron Star) as a very hungry, but small, dog. Imagine a normal Main-Sequence Star (like our Sun) as a frisky cat.

Usually, if a dog meets a cat, nothing happens. But in this specific cosmic scenario, the dog and cat get into a very specific, chaotic dance.

The paper suggests three ways this dance could happen to create the 4-hour show:

1. The "Grazing" Dance (Dynamical Channel)

The Scene: The cat and dog are wandering in a crowded neighborhood (a star cluster). They bump into each other.
The Action: The dog nips at the cat's tail (a "grazing" pass). The cat loses a little fur (mass) and runs away, but the dog is now chasing it.
The Result: The cat runs in a big circle and comes back exactly 24 hours later for a deeper bite. This second, deeper bite is the main 4-hour explosion. The first nip was the faint X-ray signal we saw a day early.

2. The "Kick" Dance (Natal-Kick Channel)

The Scene: A star explodes (Supernova) and leaves behind a newborn Black Hole.
The Action: When the star explodes, the newborn Black Hole gets a violent "kick" (like a rocket booster firing), shooting it away from its partner star. It flies through space, turns around, and crashes back into its partner.
The Result: The flight time takes about a day. The crash causes the explosion. This explains the delay perfectly.

3. The "Hybrid" Dance

A mix of the two: The Black Hole gets kicked, flies out, nips the star on the way back (the faint signal), and then comes back a day later for the main event.

Why This Theory Fits the Clues

The authors argue this "Micro-TDE" theory is the only one that explains everything:

The 4-Hour Duration: When the star gets torn apart, the debris doesn't fall in all at once. It forms a messy ring (an accretion disk) around the black hole. It's like pouring a bucket of water into a sink with a clogged drain; it trickles in slowly over hours, feeding the engine steadily.
The 24-Hour Delay: This is simply the time it takes for the star to complete one orbit around the black hole before getting torn apart again.
The "Suburbs" Location: This doesn't need to happen in the crowded center of a galaxy. It can happen anywhere two stars happen to get close, which fits the "off-center" location of GRB 250702B.
No Big Supernova: Because the star is being eaten by a small black hole (not collapsing itself), it doesn't trigger a massive, bright supernova explosion. It just gets shredded quietly.
The Fast Flickering: The explosion flickers in less than a second. This proves the engine is tiny (a stellar-mass black hole), not a giant one. It's like a small engine revving up very fast.

The Big Picture

If this theory is right, it's a huge discovery. It means we've found a new type of cosmic event where a small black hole "eats" a normal star in a slow, multi-hour feast.

It's like finding a new species of animal in the jungle. We thought we knew all the predators (collapsing stars, merging black holes), but this paper suggests there's a whole new category of "star-eating" events happening right in our galactic backyard, waiting for us to spot their faint warning signals before the main show begins.

In short: GRB 250702B wasn't a star dying; it was a star getting eaten by a small black hole in a slow-motion, 24-hour-delayed, cosmic dance.

1. Problem Statement

Ultra-Long Gamma-Ray Bursts (ULGRBs) are a rare class of high-energy transients with durations exceeding $10^3$ seconds. While standard models exist for short GRBs (compact object mergers) and long GRBs (collapsars), ULGRBs remain poorly understood.

The Specific Case: The paper focuses on GRB 250702B, a newly discovered event with unique properties that challenge existing paradigms:
- Duration: Ultra-long prompt emission ( $T_{90} \gtrsim 12$ ks, $\sim$ 4.2 hours).
- Variability: Extremely rapid $\gamma$ -ray variability ( $\delta t \sim 0.5$ s) and hard spectra extending to $\gtrsim 10$ MeV.
- Pre-peak Signal: A soft X-ray signal detected $\sim$ 1 day prior to the main burst.
- Location: Significantly offset ( $\sim$ 5.7 kpc) from the host galaxy nucleus.
- Supernova: No accompanying luminous supernova was detected.
The Conflict: Standard ULGRB progenitors fail to explain this specific combination:
- Blue-supergiant collapsars typically produce bright supernovae and lack day-scale pre-peak signals.
- Magnetar engines struggle to explain the day-scale delay and pre-peak emission.
- Intermediate-Mass Black Hole (IMBH) TDEs (e.g., White Dwarf disruptions) generally occur in galactic nuclei, have much shorter timescales ( $t_{min}, t_{acc} \sim 10-100$ s), and lack the required hard $\gamma$ -ray variability.

2. Methodology

The authors propose and analyze the Micro-Tidal Disruption Event ( $\mu$ TDE) hypothesis. In this scenario, a stellar-mass black hole (BH) or neutron star (NS) tidally disrupts a main-sequence (MS) star. The study involves:

Analytic Modeling: Deriving scaling relations for tidal radii, fallback timescales ( $t_{min}$ ), and viscous accretion timescales ( $t_{acc}$ ) for stellar-mass compact objects disrupting MS stars.
Channel Analysis: Evaluating three specific formation pathways for $\mu$ $μ$ TDEs to explain the $\sim$ $\sim$ 1-day delay between the pre-peak and main flare:
1. Dynamical (Partial/Repeating): A grazing encounter strips mass, leaving a bound core that returns after an orbital period ( $P_{orb} \sim$ day) for a deeper disruption.
2. Natal-Kick (Full): A newborn compact object receives a kick, travels ballistically, and disrupts its companion after a delay determined by the orbital period.
3. Hybrid (Natal-Kick + Partial): A kick creates a close orbit, but the first passage is partial, with the core returning for the main flare.
Comparative Analysis: Contrasting the $\mu$ TDE predictions against alternative models (collapsars, magnetars, WD-IMBH TDEs) using observational constraints (light curves, spectra, energetics, host environment).
Rate Estimation: Calculating volumetric rates for $\mu$ TDEs based on population synthesis and dynamical studies to assess consistency with observed ULGRB rates.

3. Key Contributions

Identification of $\mu$ TDE as a Viable Progenitor: The paper establishes that the disruption of a main-sequence star by a stellar-mass compact object ( $M_\bullet \sim 10 M_\odot$ ) naturally reproduces the observed timescales ( $t_{min} \sim 1-2$ hours, $t_{acc} \sim 10^4-10^5$ s) and energetics of GRB 250702B.
Explanation of the "Day-Earlier" Pre-Peak: The authors demonstrate that the $\sim$ 1-day delay is a natural consequence of the orbital period of the surviving stellar core (in partial disruptions) or the ballistic transit time of a kicked compact object (in natal-kick scenarios).
Differentiation of Emission Mechanisms:
- Dynamical Channel: Predicts a non-thermal, faint pre-peak from the first grazing passage, followed by a steeper late-time decay ( $t^{-9/4}$ ) if a core survives.
- Natal-Kick Channel: Predicts a soft, quasi-thermal precursor from the supernova ejecta-companion collision, followed by the main flare.
Jet Breakout Constraints: The study calculates that for natal-kick scenarios, the relativistic jet must survive the supernova ejecta. The ultra-long duration of GRB 250702B satisfies the condition for successful jet breakout, acting as a selection effect for this class of events.

4. Results

Timescale Consistency: For a $10 M_\odot$ BH disrupting a solar-type star, the minimum fallback time ( $t_{min}$ ) and viscous accretion time ( $t_{acc}$ ) align perfectly with the observed $\sim$ 4-hour duration and the $\sim$ 1-day pre-peak delay.
Energetics: The required accreted mass ( $\sim 0.35 M_\odot$ ) is consistent with the partial/full disruption of a main-sequence star. The jet efficiency ( $\eta_j \sim 10^{-3}$ ) matches values seen in other relativistic TDEs.
Variability and Hardness: The sub-second variability and $\gtrsim 10$ MeV photons imply a bulk Lorentz factor $\Gamma \gtrsim 10-100$ , consistent with a relativistic jet launched by a stellar-mass compact object. This rules out IMBH TDEs (where variability is limited by the larger Schwarzschild radius) and supports the stellar-mass origin.
Host Environment: The off-nuclear location is consistent with $\mu$ TDEs occurring in stellar clusters or wide binaries, whereas IMBH TDEs are expected in galactic nuclei. The lack of a bright supernova is consistent with the $\mu$ TDE scenario (especially in the dynamical channel or if the SN was obscured/weak).
Rate Estimates: The estimated volumetric rate for $\mu$ TDEs ( $\sim 100-450$ Gpc $^{-3}$ yr $^{-1}$ ) is comparable to or exceeds the inferred rate of on-axis ULGRBs, making this a statistically plausible channel.

5. Significance

First Evidence for $\mu$ TDEs: If GRB 250702B is confirmed as a $\mu$ TDE, it represents the first substantial observational evidence for this class of events, bridging the gap between standard GRBs and relativistic TDEs (like Sw1644+57).
New Progenitor Channel: It expands the taxonomy of high-energy transients beyond the standard "collapsar" and "merger" dichotomy, introducing stellar-mass compact object interactions with main-sequence stars as a distinct engine.
Observational Diagnostics: The paper provides specific discriminants for future observations:
- Pre-peak signals: Soft X-ray flares $\sim$ 1 day prior can serve as early alerts.
- Light curve slopes: Distinguishing between $t^{-5/3}$ (full disruption) and steeper slopes (partial disruption).
- Host Galaxy Type: Detection in early-type (quiescent) galaxies would strongly favor the dynamical $\mu$ TDE channel over collapsars.
Astrophysical Implications: Confirming this channel would open a new window for studying black hole demographics, natal kick physics, and stellar dynamics in dense environments (clusters and binaries) on AU scales.

In conclusion, the authors argue that GRB 250702B is best explained as a micro-tidal disruption event, offering a unified framework that resolves the conflicting observational properties (duration, delay, location, and lack of SN) that standard models cannot simultaneously satisfy.