Wind-confined jet collimation revealed by the acceleration-phase photosphere of GRB 220426A

We analyze the prompt emission of the exceptionally bright GRB 220426A observed by Fermi/GBM, whose time-resolved spectra are among the narrowest measured in any GRB. Here we show that observations during the first ∼ 5 s are consistent with the signal being emitted while the jet was still in the initial radiation-dominated acceleration phase. The time-resolved spectra allow the effective launch radius, r0, to be inferred with unusual precision. We find r0 ∼ few × 1010 cm, increasing linearly with time. These findings match the theoretical predictions of the recollimation shock, suggesting that this GRB shows the first clear evidence for the existence and evolution of a recollimation shock. Using this interpretation, the linear increase observed is sustained over a period longer than expected from a pure jet breakout. Therefore, the jet collimation must have persisted even after breakout. We suggest that the collimation was maintained by a finite, dense, wind-like circumburst medium, which would reproduce the observed behavior of the prompt emission. We conclude that late-stage progenitor mass-loss can shape the earliest prompt emission and that the photospheric emission provides a way to probe both the jet collimation and the innermost region of the circumburst medium (CBM) surrounding the progenitor, independently of the constraints from interacting supernovae.

Improvements to AI Systems:

1. Time-Resolved Spectral Inference Engine

• Improvement: Train a transformer-based model on synthetic GRB spectra generated from radiation-hydrodynamic simulations of jet acceleration phases, incorporating recollimation shock physics.

• Capability: The AI can automatically classify whether a GRB’s early prompt emission originates from the radiation-dominated acceleration phase vs. the coasting phase, and infer the effective launch radius r 0 and its temporal evolution with Bayesian uncertainty quantification—without manual spectral fitting.

2. Dynamic Jet-Collimation Model Fitting

• Improvement: Develop a neural ODE (neural ordinary differential equation) that learns the coupled dynamics of jet Lorentz factor, photon escape, and recollimation shock position as a function of time, conditioned on the circumburst medium (CBM) density profile.

• Capability: The AI can predict whether a linear increase in r 0 (as observed) is sustainable beyond jet breakout, and automatically estimate the CBM wind-like density slope and mass-loss rate from the progenitor, directly from light curves and spectral evolution.

3. Cross-Domain Progenitor Mass-Loss Inference

• Improvement: Create a multimodal AI that fuses GRB prompt emission data (from Fermi/GBM) with supernova interaction constraints (e.g., from radio/X-ray afterglows) to jointly infer the late-stage mass-loss history of the progenitor.

• Capability: The system can disentangle whether a GRB’s early photospheric emission is shaped by jet collimation vs. external CBM density, and provide independent estimates of the progenitor’s wind mass-loss rate—even when supernova signatures are absent.

4. Real-Time Anomaly Detection for Narrow Spectral Features

• Improvement: Implement an unsupervised anomaly detector (e.g., a variational autoencoder) trained on the full Fermi/GBM spectral catalog to identify GRBs with unusually narrow time-resolved spectra (like GRB 220426A).

• Capability: The AI can trigger follow-up observations and automated physical interpretation (e.g., recollimation shock candidate) in real time, prioritizing rare events for deeper multi-wavelength study.

5. Simulation-to-Observation Transfer Learning

• Improvement: Use a domain-adversarial neural network to map synthetic spectra from relativistic jet simulations (with varying r 0, CBM density, and shock evolution) to observed Fermi/GBM data, overcoming detector response and background noise.

• Capability: The AI can invert observed spectra to recover the full time-dependent geometry of the jet (opening angle, recollimation radius) and the CBM density profile, even for low-signal-to-noise bursts, enabling population-level studies of jet collimation physics.

What the Improved AI System Can Do (Specific Use Cases):

• Automatically scan all Fermi/GBM bursts to identify other candidates with recollimation shock signatures, without human spectral fitting.

• Predict the exact time window during which a GRB’s photospheric emission is sensitive to the CBM, and output the inferred wind density and mass-loss rate for each burst.

• Generate synthetic light curves and spectra for hypothetical progenitor mass-loss scenarios, allowing rapid testing of theoretical models against real data.

• Provide a probabilistic map of the jet’s launch radius evolution, flagging deviations from linear growth that might indicate jet breakout or external density discontinuities.

• Integrate with supernova searches to cross-validate mass-loss histories, reducing false positives in GRB–SN associations.

We analyze the prompt emission of the exceptionally bright GRB 220426A observed by Fermi/GBM, whose time-resolved spectra are among the narrowest measured in any GRB. Here we show that observations during the first about 5 s are consistent with the signal being emitted while the jet was still in the initial radiation-dominated acceleration phase. The time-resolved spectra allow the effective launch radius, r 0, to be inferred with unusual precision. We find r 0 about few times 10 10, cm, increasing linearly with time. These findings match the theoretical predictions of the recollimation shock, suggesting that this GRB shows the first clear evidence for the existence and evolution of a recollimation shock. Using this interpretation, the linear increase observed is sustained over a period longer than expected from a pure jet breakout. Therefore, the jet collimation must have persisted even after breakout. We suggest that the collimation was maintained by a finite, dense, wind-like circumburst medium, which would reproduce the observed behavior of the prompt emission. We conclude that late-stage progenitor mass-loss can shape the earliest prompt emission and that the photospheric emission provides a way to probe both the jet collimation and the innermost region of the circumburst medium (CBM) surrounding the progenitor, independently of the constraints from interacting supernovae.