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GRB 250424A: A Case Study of Energy Injection with Multiwavelength Observations

This paper presents a comprehensive multiwavelength analysis of the long-duration GRB 250424A, demonstrating that its afterglow evolution is best explained by a relativistic forward shock model with continuous energy injection lasting approximately 9 ks, while late-time observations constrain but do not rule out an associated supernova.

Original authors: Yifang Liang, Yun Wang, WeiKang Zheng, Priyadarshini Gokuldass, Huali Li, Chenwei Wang, Riccardo Brivio, Donovan Schlekat, Alexei V. Filippenko, Pillas Marion, Dalya Akl, Sarah Antier, Manasanun Tanas

Published 2026-06-17

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CC BY 4.0

Original authors: Yifang Liang, Yun Wang, WeiKang Zheng, Priyadarshini Gokuldass, Huali Li, Chenwei Wang, Riccardo Brivio, Donovan Schlekat, Alexei V. Filippenko, Pillas Marion, Dalya Akl, Sarah Antier, Manasanun Tanasan, Kanthanakorn Noysena, Di Xiao, Jie An, Thomas G. Brink, Krittapas Chanchaiworawit, Dylan A. Dutton, Matteo Ferro, Michael Freeberg, Ren Jia, Alain Klotz, Ye Li, Xing Liu, Dan Reichart, Antonio C. Rodriguez, Wenjun Tan, Samaporn Tinyanont, Jing Wang, Zi-Qi Wang, Jianyan Wei, Samuel E. Whitebook, Xuefeng Wu, Dong Xu, Yi Yang, Jinpeng Zhang, Wenlong Zhang, Hao Zhou, Valerio D'Elia, Massimiliano De Pasquale, Dino Fugazza, Luciano Nicastro, Damien Turpin, Roger Hellot, Frederic Dux, Xiangyu Wang, Frederic Daigne, Yongfeng Huang, HongBo Cai, Alexis Coleiro, Bertrand Cordier, Stefano Crepaldi, YongWei Dong, Olivier Godet, XuHui Han, Frederic Piron, YuLei Qiu, Stephane Schanne, Chao Wu, LiPing Xin, Yang Xu, Shuangnan Zhang, ShiJie Zheng

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. Occasionally, a massive, cosmic "tsunami" rips through it. These are Gamma-Ray Bursts (GRBs), the most powerful explosions in the universe, often caused by the death of a giant star.

This paper is a detailed case study of one specific tsunami, named GRB 250424A, which happened on April 24, 2025. The authors acted like a team of cosmic detectives, using a fleet of space telescopes and ground-based cameras to track the explosion from its first blinding flash to its fading echo.

Here is the story of what they found, explained simply:

1. The "Flat" Afterglow (The Energy Injection)

Usually, when a firework explodes, it gets bright, then fades away quickly and predictably. If you graph the brightness over time, it looks like a steep slide going down.

However, GRB 250424A did something strange. After the initial explosion, the light didn't just slide down; it hit a flat plateau. For about 2.5 hours, the X-ray and optical light stayed almost the same brightness, refusing to fade.

The Analogy: Imagine you are pushing a heavy sled down a snowy hill. Normally, friction would slow it down quickly. But in this case, it was as if someone kept pushing the sled from behind while it was sliding. This "push" kept the energy high and prevented the light from fading as fast as expected.
The Cause: The scientists concluded that the "engine" at the center of the explosion (likely a rapidly spinning dead star or a black hole) didn't shut off immediately. Instead, it kept pumping fresh energy into the explosion for hours, "refreshing" the shockwave.

2. The "Standard" Fade (The Engine Turns Off)

Eventually, the extra pushing stopped. Once the "energy injection" ceased, the light curve finally dropped off a cliff, following the standard, steep slide that most explosions follow. This confirmed that the explosion was indeed being powered by a central engine that eventually ran out of steam.

3. The Hidden Ghost (The Missing Supernova)

When massive stars die in these types of explosions, they usually leave behind a brilliant, expanding cloud of debris called a supernova. It's like the "body" of the star exploding outward.

The Mystery: The team looked very carefully at the explosion site for weeks, using powerful telescopes to see if this "body" (the supernova) appeared. They found nothing. The light curve showed no sign of the usual supernova glow.
The Explanation: Did the star die without a supernova? Not necessarily. The team realized the explosion happened inside a very dusty, messy neighborhood (the host galaxy).
The Analogy: Imagine a bright lightbulb (the supernova) is turned on inside a room filled with thick, black smoke. You can't see the lightbulb, not because it's off, but because the smoke is blocking the view.
The Conclusion: The team calculated that the dust in that galaxy was thick enough to hide a supernova that would normally be very bright. So, the supernova was likely there, but it was hidden in plain sight by cosmic dust.

4. The Cosmic Map

By studying how the light changed color and brightness, the team mapped out the environment:

The Surroundings: The explosion happened in a region of space with a steady, uniform density of gas (like a calm ocean), rather than a windy, chaotic one.
The Power: The explosion was incredibly energetic, releasing as much energy in a few seconds as our Sun would in its entire 10-billion-year lifetime.

Summary

This paper tells the story of a cosmic explosion that behaved like a sledgehammer that kept getting hit by a second hammer for a few hours, keeping the light bright longer than expected. It also taught us that sometimes, the "aftermath" of a star's death (the supernova) is invisible not because it didn't happen, but because the universe is too dusty to let us see it.

The researchers used data from the SVOM (a Chinese-French satellite) and Swift (a NASA satellite), along with many ground-based telescopes, to piece together this puzzle, confirming that even in the violent death of a star, the environment plays a huge role in what we see.

Technical Summary of GRB 250424A: A Case Study of Energy Injection with Multiwavelength Observations

Problem Statement
Gamma-ray bursts (GRBs) are the most luminous electromagnetic explosions in the universe, typically associated with the core collapse of massive stars. While the standard relativistic blast-wave model successfully explains many aspects of GRB afterglows, observations over the past two decades have revealed complex evolutionary phases, particularly the "plateau" or shallow-decay phase in X-ray light curves. While X-ray plateaus are common, achromatic plateaus (simultaneous in X-ray and optical) are rare and challenge standard interpretations. Distinguishing between competing physical scenarios for these plateaus—specifically continuous energy injection from a long-lived central engine versus hydrodynamical "refreshed shocks" from stratified ejecta—requires rigorous broadband modeling to test closure relations between temporal decay indices ( $\alpha$ ) and spectral indices ( $\beta$ ). Additionally, the association between long GRBs and broad-lined Type Ic supernovae (SNe Ic-BL) is well-established, yet the detectability of the supernova component varies significantly, necessitating deep late-time monitoring to constrain explosion energies and progenitor diversity.

Methodology
The authors present a comprehensive multiwavelength analysis of the long-duration GRB 250424A, detected on 2025 April 24. The study utilizes a dataset spanning from prompt gamma-ray emission to late-time optical spectroscopy.

Observations: Prompt emission was characterized using data from the SVOM/GRM (15 keV–5 MeV) and Swift/BAT (15–350 keV). Afterglow monitoring was conducted across X-ray, optical, and near-infrared (NIR) bands using Swift/XRT, SVOM/VT, the GRANDMA consortium (including TRT, TAROT, Euler, REM, and Skynet telescopes), and the Keck I 10m telescope.
Spectroscopy: Two epochs of optical spectroscopy were obtained with Keck/LRIS at $T_0 + 26$ days and $T_0 + 59$ days (observer frame) to measure redshift and search for supernova features.
Analysis: The team performed joint temporal and spectral analysis of the prompt emission. For the afterglow, they constructed broadband spectral energy distributions (SEDs) at five representative epochs. They applied a smoothly broken power-law (SBPL) model to the light curves and tested standard synchrotron closure relations.
Modeling: To constrain physical parameters, the authors employed the ASGARD numerical code, which solves relativistic hydrodynamics and time-dependent electron energy distribution equations, self-consistently calculating synchrotron and synchrotron self-Compton (SSC) emission. They utilized Markov Chain Monte Carlo (MCMC) fitting to derive posterior distributions for jet and environmental parameters.

Key Results

Prompt Emission: GRB 250424A is a long-duration burst ( $T_{90} \approx 19.5$ s) with a single-pulse morphology. The spectrum is well-modeled by a cutoff power-law (CPL) with a peak energy $E_p \approx 143$ keV (rest frame) and an isotropic equivalent radiated energy $E_{\gamma,iso} \approx 1.7 \times 10^{52}$ erg. The burst follows the Amati relation, consistent with typical long GRBs.
Afterglow Evolution: The X-ray and optical/NIR light curves exhibit a synchronized, achromatic evolution. They display a prominent shallow decay phase (Phase A) lasting until $t \approx 10^4$ $t \approx 1 0^{4}$ s, followed by a transition to a standard decay regime (Phase B).
- Phase A: Decay indices are $\alpha_X \approx 0.23$ and $\alpha_{opt} \approx 0.34$ .
- Phase B: Decay indices steepen to $\alpha_X \approx 1.07$ and $\alpha_{opt} \approx 0.97$ .
- Spectral Behavior: Broadband SEDs at all epochs are well-described by a single power-law ( $\beta_{OX} \approx 0.6-0.7$ ), indicating the optical and X-ray emission originate from the same synchrotron segment ( $\nu_m < \nu < \nu_c$ ) with no cooling break crossing.
Physical Interpretation:
- Energy Injection: The shallow decay is successfully reproduced by a model of continuous energy injection into a forward shock propagating in a constant-density interstellar medium (ISM). The injection phase lasts from $\sim 200$ s to $\sim 8,000$ s with an injection index $q \approx 0.34$ (numerical modeling) to $0.5$ (analytical closure relations).
- Parameters: The isotropic equivalent kinetic energy is $E_{K,iso} \approx 5.5 \times 10^{52}$ erg, implying a radiative efficiency of $\sim 27\%$ . The initial bulk Lorentz factor is $\Gamma_0 \approx 550$ . The circumburst density is $n_0 \approx 2.6$ cm $^{-3}$ . The microphysical parameters are $\epsilon_e \approx 0.06$ and $\epsilon_B \approx 10^{-4}$ .
- Jet Geometry: No achromatic jet break was detected within the observational window, yielding a lower limit on the jet opening angle of $\theta_j \gtrsim 11^\circ$ .
Supernova and Host Galaxy:
- Supernova: Despite deep late-time monitoring and spectroscopy, no distinct supernova component was detected. However, SED analysis reveals significant host-galaxy extinction ( $E(B-V)_{host} \approx 0.47$ mag, $A_V \approx 1.5$ mag). The authors demonstrate that a prototypical SN 1998bw-like event, attenuated by this extinction, would fall below the detection limits of their campaign. Thus, the non-detection is consistent with a standard collapsar scenario obscured by dust.
- Host Galaxy: The host is a star-forming galaxy at $z=0.31$ with low metallicity ( $Z \approx 0.07 Z_\odot$ ) and a star-formation rate of $1.6 M_\odot$ yr $^{-1}$ .

Significance and Claims
The paper claims that GRB 250424A serves as a "textbook" example of an afterglow dominated by sustained energy injection. The primary significance lies in the simultaneous, achromatic nature of the plateau in both X-ray and optical bands, which strongly supports a common synchrotron origin and rules out spectral break crossings as the cause of the temporal break. The study demonstrates that the observed temporal and spectral behavior can be self-consistently explained within the external shock framework with energy injection, bridging analytical closure relations and detailed numerical modeling. Furthermore, the work highlights the critical role of local environmental obscuration (dust extinction) in GRB-SN studies, showing that the absence of a detected supernova does not necessarily imply a deviation from the standard collapsar model. The high-quality data from SVOM, Swift, and ground-based facilities allowed the authors to isolate central-engine activity effects from geometric and environmental factors, providing a robust constraint on the diversity of central-engine lifetimes.

1. The "Flat" Afterglow (The Energy Injection)

2. The "Standard" Fade (The Engine Turns Off)

3. The Hidden Ghost (The Missing Supernova)

4. The Cosmic Map

Summary

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