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Can GRB 250702B be explained as the tidal disruption of a white dwarf by an intermediate mass black hole? Yes

The paper proposes that the repeating gamma-ray transient GRB 250702B is caused by a white dwarf undergoing multiple tidal stripping episodes by an intermediate-mass black hole, where chaotic debris collisions upscatter X-rays to produce the observed gamma-ray emission while the resulting accretion powers the jet and other multi-wavelength counterparts.

Original authors: Rob AJ Eyles-Ferris, Andrew King, Rhaana LC Starling, Peter G Jonker, Andrew J Levan, Antonio Martin-Carrillo, Tanmoy Laskar, Jillian C Rastinejad, Nikhil Sarin, Nial R Tanvir, Benjamin P Gompertz, Nu

Published 2026-01-28

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CC BY 4.0

Original authors: Rob AJ Eyles-Ferris, Andrew King, Rhaana LC Starling, Peter G Jonker, Andrew J Levan, Antonio Martin-Carrillo, Tanmoy Laskar, Jillian C Rastinejad, Nikhil Sarin, Nial R Tanvir, Benjamin P Gompertz, Nusrin Habeeb, Paul T O'Brien, Massimiliano De Pasquale

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 Mystery of the "Repeating" Explosion

Imagine you are watching the night sky, and suddenly, a massive explosion of gamma rays (the most energetic light in the universe) happens. Usually, these explosions—called Gamma-Ray Bursts (GRBs)—are like a single firework: they go off once and are done.

But GRB 250702B was weird. It didn't just go off once; it triggered a detector three separate times over the course of a few hours. It was like a firework that kept popping, then paused, then popped again. Scientists were baffled because standard models of exploding stars or merging neutron stars can't explain this "repeating" behavior.

The Solution: A Cosmic Dance of Destruction

The authors of this paper propose a specific scenario to explain the mystery: A White Dwarf star getting slowly eaten by a "middle-sized" Black Hole.

To understand this, let's break down the cast of characters:

The White Dwarf: Think of this as the dense, burnt-out core of a star, like a heavy, compact marble.
The Intermediate Mass Black Hole (IMBH): This isn't a tiny black hole (like the ones left by dead stars) and not a supermassive one (like the ones at the center of galaxies). Think of it as a "Goldilocks" black hole—large enough to be dangerous, but small enough to be picky.
The Orbit: Instead of falling straight in and dying instantly, the White Dwarf is in a tight, elliptical orbit around the Black Hole, like a planet orbiting a star, but much faster and closer.

The "Peeling" Process (The Repeating Triggers)

Here is the core of the theory, explained with a simple analogy:

Imagine the White Dwarf is a hard candy and the Black Hole is a giant, hungry mouth.

The First Bite: As the candy swings close to the mouth, the Black Hole's gravity grabs the outer layers. It doesn't swallow the whole thing yet; it just peels off a little bit of the surface.
The Expansion: Once the candy loses some mass, it actually gets bigger (like a balloon expanding when you let some air out, but in reverse physics). Because it's now bigger, the next time it swings close, the Black Hole grabs more of it.
The Runaway Effect: This happens over and over. The star gets bigger, the Black Hole grabs more, the star gets even bigger. This is the "runaway" effect.
The Final Crunch: Eventually, the star gets so big and weak that the Black Hole rips it apart completely.

Why the repeating triggers?
Every time the star swings close (pericentre passage), a little bit of debris is stripped off. These debris streams crash into each other, creating a shockwave.

The X-rays: The Black Hole eats this debris, creating a bright, steady glow of X-rays (the "precursor" seen before the big explosion).
The Gamma Rays: The debris streams crash into each other like two cars colliding on a highway. This collision creates a flash of light. Because the Black Hole is shooting out a jet of particles (like a high-speed water hose), it hits these flashes and boosts their energy, turning them into the high-energy Gamma Rays we see.

Since the star swings by the Black Hole at regular intervals, these "crashes" happen at regular intervals, explaining the three repeating triggers.

Explaining the Other Clues

The paper checks if this story fits all the other evidence they found:

The Fading X-rays: After the final destruction, the X-rays fade away quickly. The model explains this because the star was in a very stretched-out (eccentric) orbit, causing the debris to fall in and fade faster than a normal star would.
The Radio and Infrared: The "water hose" jet (the relativistic jet) crashes into the gas surrounding the Black Hole. This creates a shockwave that glows in radio waves (like a sonic boom) and infrared light (heat).
The Speed: The gamma rays change brightness in seconds. This fits the model because the debris streams are chaotic and collide very quickly, creating rapid flashes.

Is This Scenario Likely?

The authors admit this is a rare event. It requires a specific setup:

A White Dwarf must be in a binary system (a pair of stars).
A Black Hole must pass by and steal the White Dwarf, leaving it trapped in a tight orbit (a process called "Hills capture").
The orbit must be just right to allow for multiple "peelings" before the final destruction.

They calculate that while this is rare, it's not impossible. Given that we have only seen one of these in 17 years of monitoring, the math suggests this is a plausible, albeit uncommon, cosmic event.

The Bottom Line

The paper concludes that GRB 250702B is likely the story of a White Dwarf star being slowly stripped apart by a medium-sized Black Hole. The repeating gamma-ray flashes are the result of the star's debris crashing into itself as it gets torn apart, with the Black Hole's jet acting like a giant magnifying glass to boost the energy of the light we see. It's a unique "tidal disruption event" that solves the mystery of why this explosion happened in three distinct bursts.

Technical Summary of "Can GRB 250702B be explained as the tidal disruption of a white dwarf by an intermediate mass black hole?"

Problem Statement
GRB 250702B presents a unique astrophysical transient that defies standard high-energy transient models. Its defining characteristics include: (i) repeating gamma-ray triggers over several hours with near-constant intervals (~2825 s in the observer frame); (ii) a precursor X-ray counterpart detected a day prior to the gamma-ray flares, which rose and faded rapidly; (iii) fast-fading near-infrared (NIR) and long-lived radio counterparts; and (iv) gamma-ray emission peaking in the MeV range with second-scale variability. While previous analyses suggested possibilities ranging from micro-TDEs (main sequence star disrupted by a stellar-mass black hole) to compact object mergers, the specific combination of periodicity, precursor behavior, and energetics (isotropic equivalent energy $\sim 2 \times 10^{54}$ erg at $z=1.036$ ) remains ambiguous. The central problem addressed is whether a White Dwarf (WD) tidally disrupted by an Intermediate Mass Black Hole (IMBH) can simultaneously explain these multi-wavelength features.

Methodology
The authors re-evaluate the nature of GRB 250702B using newly available redshift data ( $z=1.036 \pm 0.004$ ) from the James Webb Space Telescope (JWST) to constrain energetics and X-ray properties.

X-ray Data Analysis: The team re-analyzed spectral and timing data from Swift-XRT and two Chandra Deep Drilling Time (DDT) observations (ObsIDs 31011 and 31468). They fitted the data with an absorbed power-law model, accounting for both Galactic and host-galaxy absorption, to derive luminosities and spectral indices.
Modeling Framework: The authors propose a model where a WD is bound to an IMBH ( $M_{\bullet} \sim 10^4 M_{\odot}$ $M_{∙} \sim 1 0^{4} M_{⊙}$ ) and undergoes multiple pericentre passages before final disruption.
- Accretion Physics: They utilize a semi-analytical approximation for the fallback rate of debris, moving beyond the "frozen-in" approximation to account for tidal distortion and self-gravity effects, fitting this to numerical simulation results (Cufari et al. 2022a).
- Jet Dynamics: They assume a mildly relativistic, wide-angle jet powered by super-Eddington accretion. The X-ray emission is attributed to internal jet processes.
- Gamma-ray Mechanism: To explain the MeV gamma-rays, the authors propose that chaotic debris streams from repeated stripping episodes collide near pericentre. These collisions produce thermal X-ray photons which are then upscattered to MeV energies via Inverse Compton scattering by the relativistic jet.
Statistical Fitting: The model parameters (black hole mass, WD mass, orbital eccentricity, and radiative efficiency) were constrained using a Markov Chain Monte Carlo (MCMC) sampler (emcee) against the long-term X-ray light curve.

Key Contributions and Results

X-ray Light Curve Reproduction: The WD-IMBH TDE model successfully reproduces the long-term X-ray decay ( $L_X \propto t^{-1.9}$ ) observed by Swift, EP, and Chandra. The fitted parameters yield a black hole mass of $\log(M_{\bullet}/M_{\odot}) \approx 4.02 \pm 0.63$ and a WD mass of $0.45 \pm 0.20 M_{\odot}$ . The model predicts a jet half-opening angle of $\theta_j \sim 0.07 - 0.40$ rad and a Lorentz factor $\Gamma \sim 2.5 - 13.7$ .
Explanation of Periodicity: The model posits that the WD is on a highly eccentric, bound orbit ( $e \approx 0.92$ ), likely formed via Hills capture. The periodic gamma-ray triggers correspond to the orbital period, during which debris streams from previous stripping episodes collide. The authors note that not every passage results in a collision due to stream precession, consistent with the observed spacing of the three triggers.
Gamma-ray Origin: The paper demonstrates that the jet alone (with low $\Gamma$ ) cannot produce the observed MeV gamma-rays. Instead, the authors show that stream-stream collisions near pericentre generate thermal X-ray flashes ( $\sim 15$ keV) with durations of $\sim 0.03$ s. These photons are upscattered by the jet to the observed MeV energies. The kinetic energy budget of the jet is sufficient to power this upscattering, even if it results in an overprediction of the direct X-ray luminosity (suggesting energy loss to the gamma-ray channel).
Multi-wavelength Consistency: The model accounts for the precursor X-ray emission (repeated stripping prior to full disruption), the fast-fading NIR (thermal emission from the accretion flow), and the radio emission (synchrotron from external shocks).

Significance and Claims
The authors claim that the WD-IMBH TDE model is a viable and comprehensive explanation for GRB 250702B, reconciling features that are difficult to explain with standard collapsar or merger models. Specifically:

The model naturally explains the repeating nature and periodicity of the event through the bound, eccentric orbit of the WD.
It resolves the energy budget discrepancy by introducing a two-stage emission mechanism: thermal emission from stream collisions upscattered by the jet to produce gamma-rays, while the jet's internal processes power the X-rays.
The derived system parameters (IMBH mass, WD mass, eccentricity) are physically consistent with formation scenarios involving the Hills mechanism.

The paper maintains a modest stance, acknowledging that while the model fits the data well, it is not the only feasible interpretation (e.g., micro-TDEs or mergers remain possibilities). The authors emphasize that future analysis of jet properties and further observations are required to place firmer constraints on the model. They estimate the volumetric rate of such events to be low ( $< 10^{-3} \text{ Gpc}^{-3} \text{ yr}^{-1}$ ), consistent with the rarity of GRB 250702B.