Probing TeV Afterglow Emission of GRB~221009A with Gaussian Structured jet in Wind-driven medium

TL;DR

This paper models TeV afterglow emissions of GRB 221009A using a Gaussian structured jet in a wind-driven medium, highlighting how jet geometry and environment influence high-energy photon detection.

Contribution

It introduces a Gaussian structured-jet model for TeV afterglows in wind environments, explaining observed emissions and predicting CTA detectability.

Findings

Gaussian jets produce early bright peaks and delayed softer peaks depending on observer angle.

TeV peak time and flux are sensitive to jet geometry, energy, and wind density.

Only about 10% of simulated TeV events are detectable by CTA in wind media.

Abstract

Recent detections of very high energy (VHE; GeV-TeV) photons from gamma-ray burst (GRB) afterglows, most notably the extreme event GRB 221009A, require refined models that include realistic jet structures and complex circumburst environments. The jet's angular structure is crucial for shaping afterglow emission. Our recent work demonstrates that Gaussian jets, with their smooth angular decline, naturally produce early bright peaks for on-axis observers and delayed, softer, dimmer peaks at higher inclinations. The gradual decline suppresses excessive lateral expansion, unlike the sharp edge in top-hat jets, making Gaussian jets a compelling alternative to both top-hat and other structured-jet models. Here we implement a Gaussian structured-jet model to explain TeV afterglows from adiabatic forward shocks propagating in a wind-driven medium. We show that the TeV peak time and flux depend sensitively on jet geometry, kinetic energy, wind density, and on microphysical parameter ratios that scale the SSC component. We identify the afterglow parameter space that is favourable for detecting sub-TeV photons with the Cherenkov Telescope Array (CTA), finding that only about ten per cent of simulated TeV events exceed CTA sensitivity in a wind medium. These detections arise from near core-aligned views, with high kinetic energy and wind density, moderate initial Lorentz factor and downstream magnetic field, and a relatively large fraction of energy in nonthermal electrons. Applying this model to GRB 221009A, we perform multi-band fits including wind-modified dynamics, Klein-Nishina effects, and EBL attenuation, and find that a mildly off-axis geometry reproduces the observed X-ray and GeV-TeV light curves.