GRB 221009A Afterglow from a Shallow Angular Structured Jet

Abstract

Exceptionally bright gamma-ray burst (GRB) afterglows can reveal the angular structure of their jets. GRB jets appear to have a narrow core (of half-opening angle ${\theta }_c$), beyond which their kinetic energy drops as a power-law with angle $\theta$ from the jet's symmetry axis, $E_{k,\mathrm{iso}}(\theta )\propto [1+(\theta /{\theta }_c{)}^2{]}^{-a/2}$. The power-law index $a$ reflects the amount of mixing between the shocked jet and confining medium, which depends on the jet's initial magnetization. Weakly magnetized jets undergo significant mixing, leading to shallow ($a\lesssim 2$) angular profiles. We use the exquisite multi-waveband afterglow observations of GRB 221009A to constrain the jet angular structure using a dynamical model that accounts for both the forward and reverse shocks, for a power-law external density profile, $n_{\mathrm{ext}}\propto R^{-k}$. Both the forward-shock emission, that dominates the optical and X-ray flux, and the reverse-shock emission, that produces the radio afterglow, require a jet with a narrow core (${\theta }_c\approx 0.021$) and a shallow angular structure ($a\approx 0.8$) expanding into a stellar wind ($k\approx 2$). Moreover, these data appear to favor a small fraction (${\xi }_e\approx 10^{-2}$) of shock-heated electrons forming a power-law energy distribution in both shocks.

arXiv comment5 pages, 3 figures. Accepted to MNRAS Letters

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