A Narrow Uniform Core with a Wide Structured Wing: Modeling the TeV and Multi-wavelength Afterglows of GRB 221009A

TL;DR

This paper models the multi-wavelength afterglows of GRB 221009A using a two-component jet with a stratified density profile, successfully explaining observations from TeV to X-ray and optical wavelengths.

Contribution

It introduces a two-component jet model with a stratified density profile to simultaneously explain multi-wavelength afterglows of GRB 221009A, addressing previous limitations.

Findings

A decreasing density profile is favored for the burst environment.

The two-component jet model explains TeV, X-ray, and optical afterglows.

Radio flux discrepancies suggest non-standard microphysics may be needed.

Abstract

The TeV afterglow of the BOAT GRB 221009A was interpreted as arising from a narrow jet while the radio to X-ray afterglows were interpreted as arising from a wide structured jet. However, there is no model explaining the TeV and lower-energy multi-wavelength afterglows simultaneously. We here investigate a two-component jet model, including a narrow uniform core with a wide structured wing, to explain both the multi-wavelength afterglows that last up to 100 days. We find that to explain the early TeV afterglow with the inverse-Compton process, we need a circum-burst density higher than $\gtrsim 0.1{\rm cm^{-3}}$, while the radio afterglow and the H.E.S.S. upper limit combine to constrain the density to be lower at larger radii. Thus, a decreasing density profile with radius is favored. Considering that the rising TeV light curve during the afterglow onset favors a constant-density medium, we invoke a stratified density profile, including a constant-density profile at small radii and a wind density profile at large radii. We find that the two-component jet model with such a stratified density profile can explain the TeV, X-ray and optical afterglows of GRB 221009A, although the radio fluxes exceed the observed ones by a factor of two at later epochs. The discrepancy in the radio afterglow could be resolved by invoking some non-standard assumption about the microphysics of afterglow shocks. The total kinetic energy of the two components in our model is $\lesssim 10^{52}{\rm erg}$, significantly smaller than that in the single structured jet models.