Multi Messenger Study of GRB 221009A with VHE Gamma-ray and Neutrino Afterglow from a Gaussian Structured Jet

TL;DR

This study models the very-high-energy gamma-ray and neutrino afterglow of GRB 221009A using a Gaussian structured jet, revealing insights into jet structure, emission mechanisms, and neutrino detectability with upcoming telescopes.

Contribution

It introduces a Gaussian structured jet model for GRB 221009A's VHE afterglow and assesses neutrino fluxes, providing new constraints on jet geometry and emission processes.

Findings

Gaussian jet model reproduces TeV emission without large energies.

Predicted neutrino fluxes are below current detector sensitivities.

Jet orientation significantly affects neutrino signal strength.

Abstract

Recent detections of very-high-energy (VHE; $\gtrsim 100$~GeV) emission from GRB afterglows, notably the unprecedented brightness of GRB~221009A observed by LHAASO, reveal emission components beyond the standard electron synchrotron model. Multi-TeV photons motivate synchrotron self-Compton and possibly hadronic contributions, while the non-detection of coincident neutrinos by IceCube/KM3NeT/GRAND200k constrains the microphysical parameters, jet kinetic energy and ambient medium density. We model the VHE afterglow of GRB 221009A with an external forward shock from a Gaussian structured jet in a uniform density medium. This angular structure reproduces the extreme TeV output at an off-axis angle but without demanding large energies as in a top-hat jet. We also compute the corresponding $p\gamma$ neutrino flux in the PeV-EeV energies and derive a time-integrated upper limit based on the effective areas of IceCube Gen2 and GRAND200k, providing the contributions of individual GRBs to the neutrino events. The predicted neutrino flux for GRB 221009A with model parameters inferred from multi-wavelength spectral energy distribution lies below the sensitivities of these detectors. Even under highly optimistic microphysical conditions, our correlation analysis infers that the events from this GRB are of order $\sim 0.1$ for upcoming GRAND200k. We also compare neutrino fluxes for on-axis and off-axis viewing geometries and find that jet orientation alone can introduce nearly an order of magnitude variation in the signal. Thus, our studies imply that a GRB both brighter and closer than GRB 221009A would be crucial for any neutrino detections by upcoming neutrino telescopes. Future GRB detections by the CTA will provide important constraints on their geometry, radiation mechanisms, and any potential associated neutrino signals.