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Late-time JWST/NIRCam Observations of the Extremely Long-duration GRB 250702B/EP 250702a and Its Host Galaxy

This paper presents late-time JWST/NIRCam observations of the extremely long-duration GRB 250702B, characterizing its massive, dust-rich host galaxy and reporting tentative detections of the transient in specific filters that suggest a potential light curve flattening consistent with either a jetted tidal disruption event or a supernova-plus-afterglow model.

Original authors: Huei Sears, Jean J. Somalwar, Ryan Chornock, Tanmoy Laskar, Andrew Levan, Raffaella Margutti, Brendan O'Connor, Nayana A. J., Tomas Ahumada, Kate D. Alexander, Igor Andreoni, Akash Anumarlapudi, Jonat

Published 2026-06-18

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Huei Sears, Jean J. Somalwar, Ryan Chornock, Tanmoy Laskar, Andrew Levan, Raffaella Margutti, Brendan O'Connor, Nayana A. J., Tomas Ahumada, Kate D. Alexander, Igor Andreoni, Akash Anumarlapudi, Jonathan Carney, James Freeburn, Lluís Galbany, Benjamin P. Gompertz, Or Graur, Saarah Hall, Xander J. Hall, Erica Hammerstein, Saurabh W. Jha, Mansi M. Kasliwal, Dheeraj Pasham, Itai Sfaradi, Yuhan Yao

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 Big Picture: A Cosmic Mystery

Imagine the universe is a dark stage, and usually, when a massive star dies, it explodes in a brilliant, short flash of light called a Gamma-Ray Burst (GRB). Think of this like a camera flash: it's incredibly bright but lasts only a second or two.

But on July 2, 2025, astronomers saw something weird. A burst of energy named GRB 250702B didn't just flash; it kept glowing for over 110,000 seconds (more than 30 hours). It was like a camera flash that refused to turn off, shining for days. This made it an "extremely long-duration" GRB, a rare and confusing event.

The Investigation: Looking Back with a Super-Telescope

About 95 days after the event, the authors used the James Webb Space Telescope (JWST) to take a close-up picture of the spot where the burst happened. They wanted to answer two main questions:

Where did it come from? (What kind of galaxy hosted it?)
What is it actually? (Is it a dying star, or something else entirely?)

Clue #1: The Host Galaxy (The "Stage")

The burst happened inside a galaxy that looks like a giant, dusty pancake viewed from the side (edge-on).

The Dust Lane: Imagine a thick, dark belt of smoke running right through the middle of the galaxy. The burst happened right inside this dusty belt.
The Size: The galaxy is a heavyweight champion. It is one of the most massive and brightest galaxies ever seen hosting this type of event. It's like finding a tiny firecracker going off inside a massive, crowded stadium.

Clue #2: The "Ghost" Signal

The team tried to see the burst itself 95 days later. Because the burst was hidden behind that thick dust lane, it was very hard to see.

The Ambiguity: In two specific colors of light (blue and near-infrared), the telescope saw a faint, blurry spot that might be the burst. It's like trying to spot a firefly in a foggy forest; you think you see a glow, but it might just be a trick of the light or a patch of fog.
The Result: If that glow is real, the burst didn't fade away as expected. Instead, it seemed to flatten out and stay steady, like a light that stopped dimming and just stayed on.

The Three Suspects: What Caused the Burst?

The scientists are trying to figure out which "suspect" caused this long-lasting event. They have three main theories:

1. The "Super-Star" Collapse (The Standard Theory)

The Idea: A massive star collapsed into a black hole, creating a jet of energy.
The Problem: Usually, these jets fade away quickly. For this one to stay bright for so long, it would need to be accompanied by a supernova (a star explosion) that is twice as bright as the brightest ones we usually see.
The Catch: We haven't seen the "smoke" (spectral evidence) of such a massive explosion yet, though the dust might be hiding it.

2. The "Star-Eating" Black Hole (The TDE Theory)

The Idea: A star wandered too close to a black hole, got ripped apart (tidally disrupted), and the black hole started eating it, shooting out a jet.
The Fit: This theory fits the "flat light curve" (the light staying steady) very well. Usually, these events happen at the very center of a galaxy (like a lion in a cave). But this one happened off-center, in the dusty disk.
The Twist: If this is true, it would be the first time we've seen a "star-eating" event happen away from the center of a galaxy, possibly involving a smaller, "intermediate-sized" black hole rather than a giant one.

3. The "Helium Star" Merger

The Idea: A black hole crashed into a helium star.
The Status: This is a new theory, but the data doesn't rule it out. However, we don't have enough evidence to confirm it yet.

The Verdict: Still a Mystery

The paper concludes that GRB 250702B is a "chameleon."

If the faint glow they saw is real, it looks a lot like a star-eating black hole (TDE) that is smaller than usual and living in the suburbs of a galaxy, not the city center.
If the glow is just an illusion, then it might just be a dying star that is unusually bright and dusty.

The Bottom Line:
The authors say, "We need more pictures." Because the host galaxy is so messy and dusty, it's hard to tell if the faint signal is the burst or just background noise. They need to take "before" and "after" pictures (templates) to clear up the fog. Until then, GRB 250702B remains one of the universe's most enigmatic and extreme events, challenging our current rules about how stars die and how black holes behave.

Technical Summary: Late-time JWST/NIRCam Observations of the Extremely Long-duration GRB 250702B/EP 250702a and Its Host Galaxy

Problem and Context
The intrinsic nature of GRB 250702B (triggered by Fermi on July 2, 2025, with a gamma-ray duration $T_{90} \geq 25,000$ s and an X-ray duration extending to $\sim 110,000$ s) remains ambiguous. Three primary hypotheses exist: (1) an extreme collapsar-driven Long GRB (LGRB) with precursor X-ray emission; (2) a jetted Tidal Disruption Event (TDE) from an intermediate-mass black hole ( $M_{BH} \leq 10^5 M_\odot$ ) occurring off-nuclear; or (3) a merger between a black hole and a helium star. Previous observations established the transient's location within the dust lane of a host galaxy at $z = 1.036$ , but the nature of the late-time emission and the specific properties of the host galaxy required further characterization to distinguish between these progenitor scenarios.

Methodology
The authors present new JWST/NIRCam imaging of the GRB 250702B field taken at $\Delta t \approx 95$ days post-trigger (observer frame) in filters F150W, F200W, F277W, F356W, and F444W. To address contamination from a nearby bright star's diffraction spike, the team employed a template subtraction technique using a similarly bright star with no underlying host structure, scaling the template by a factor of 1.2 to match the spike intensity.

Transient photometry was derived via forced aperture and PSF photometry on GALFIT-subtracted residuals, where the host galaxy was modeled with Sérsic and edge-on disk profiles. The team also re-analyzed HST/WFC3 F160W data from $\Delta t \approx 12.5$ days, using the JWST F150W image as a template for subtraction to isolate the transient flux. Host galaxy physical parameters were constrained using the Prospector Bayesian inference framework, fitting the JWST photometry with a delayed- $\tau$ star-formation history, Calzetti attenuation law, and mass-metallicity priors. Finally, the transient light curve and host properties were compared against a sample of known ultralong GRBs (ULGRBs), jetted TDEs, and a redshift-matched sample of standard LGRB hosts.

Key Results

Host Galaxy Properties: The host is a single, massive galaxy ( $\log(M_*/M_\odot) = 11.0^{+0.2}_{-0.3}$ ) viewed nearly edge-on with a prominent dust lane. The Prospector modeling indicates a high dust column ( $A_V = 2.8 \pm 0.3$ mag). Compared to LGRB hosts at similar redshifts ( $z \sim 0.8–1.3$ ), this host is the most massive and the brightest in rest-frame $r$ and $H$ bands, despite the significant dust extinction.
Transient Detection: In the reddest filters (F277W, F356W, F444W), no transient emission was detected, consistent with upper limits. However, forced photometry in the bluer filters (F150W and F200W) reveals possible $\sim 3\sigma$ detections at magnitudes $m_{F150W} \approx 27.9$ AB mag and $m_{F200W} \approx 27.4$ AB mag.
Light Curve Behavior: If the F150W/F200W detections are secure, the light curve exhibits a flattening (plateau) at late times, deviating from the $t^{-1.8}$ power-law decay observed in X-rays. If these are non-detections (upper limits), they are consistent with the extrapolated afterglow decay but do not constrain it further.
Comparative Analysis:
- Jetted TDE Interpretation: The possible late-time plateau and the F150W–F200W color are consistent with the behavior of known jetted TDEs, though the plateau is $\sim 2$ magnitudes fainter than most (except AT 2022cmc). This faintness aligns with expectations for a TDE powered by a lower-mass black hole ( $M_{BH} \leq 10^5 M_\odot$ ). The host mass is also consistent with samples of off-nuclear TDEs.
- ULGRB Interpretation: The light curve is also consistent with a standard LGRB afterglow plus a supernova (SN) component. Modeling suggests a SN component $\sim 2\times$ brighter than SN 1998bw would be required to match the photometry, similar to the SN 2011kl associated with GRB 111209A. However, this contradicts earlier JWST/NIRSpec non-detections of a SN, though the authors note this tension is degenerate with host extinction uncertainties.

Significance and Claims
The paper posits that GRB 250702B is an extreme and enigmatic transient that challenges current classifications of collapsar GRBs and TDEs.

If a Jetted TDE: It would represent the first confirmed off-nuclear jetted TDE and the first associated with a black hole less massive than a central supermassive black hole.
If a ULGRB: It would be only the second ULGRB with an associated supernova, highlighting potential diversity in the progenitor paths of ultralong events.
Observational Ambiguity: The authors emphasize that the complex structure of the host galaxy (specifically the dust lane) and the lack of a late-time template image create ambiguity regarding the reality of the F150W/F200W detections. Consequently, the paper concludes that while the data are consistent with both a jetted TDE plateau and a GRB+SN model, definitive classification requires future panchromatic studies and, crucially, late-time template imaging with JWST/NIRCam to resolve the transient emission from the host structure.