Modelling multiwavelength afterglows of the VHE-GRB population
Monica Barnard, Ankur Ghosh, Jagdish C. Joshi, Soebur Razzaque
TL;DR
The paper presents a uniform one-zone forward-shock modelling approach for five VHE-detected GRBs, using NAIMA with MCMC to fit multiwavelength afterglow SEDs and assess ISM vs wind environments. It finds SSC-dominated VHE emission with negligible external Compton, generally favors ISM for several bursts and wind for the brightest, and reveals a population-level anti-correlation between $E_{ m k,iso}$ and $\epsilon_B$, while noting radio flux overpredictions in some epochs. The study constrains emission sites and microphysical parameters, highlighting that VHE GRBs probe low $\epsilon_B$ environments and dense media, and suggests that more complex, multi-zone or structured-jet models may be required for some events. The results provide benchmarks for current and upcoming facilities (CTA, next-gen γ-ray observatories) to refine our understanding of particle acceleration and radiation in extreme relativistic jets.
Abstract
The recent detection of very high energy (VHE, $\gtrsim$ 100 GeV) $γ$-ray emission from gamma-ray bursts (GRBs) has provided new insights into afterglow physics. Understanding the temporal and spectral evolution of VHE GRBs requires detailed modelling of multiwavelength observations spanning radio to VHE $γ$ rays. Previous studies interpreted afterglow of VHE GRBs using a range of frameworks, including single- and multi-zone jet configurations, synchrotron radiation from forward and reverse shocks, synchrotron self-Compton (SSC) processes, as well as hadronic emission processes. We have modeled five long-duration VHE GRBs - GRB 180720B, GRB 190114C, GRB 190829A, GRB 201216C and GRB 221009A; using the NAIMA code and modifications to it. The results from our analysis indicate that SSC is the dominant VHE emission mechanism, with negligible contribution from external Compton. Most VHE GRBs are well described by the forward shock model in a spherical jet configuration, where constant density interstellar medium is preferred over wind medium. Additionally, we find that VHE GRBs tend to occur in environments with lower magnetic fields and higher ambient medium densities. Interestingly, VHE GRBs lie at the edge of the $3σ$ region of the $E_{\rm k,iso}$ - $ε_B$ correlation observed in other energetic GRBs. Our model slightly over predicts the radio fluxes, indicating that a more complicated modelling might be required in some cases. These findings provide crucial constraints on VHE GRB emission sites and mechanisms and serve as a benchmark for future observations and theoretical studies in the era of CTA and next-generation $γ$-ray observatories.