http://arxiv.org/abs/2304.14331
The afterglows of exceptionally bright gamma-ray bursts (GRBs) can reveal the angular structure of their ultra-relativistic jets after they emerge from the confining medium, e.g. the progenitor’s stellar envelope in long-soft GRBs. These jets appear to have a narrow core (of half-opening angle $\theta_c$), beyond which their kinetic energy drops as a power-law with the angle $\theta$ from the jet’s symmetry axis, $E_{k,\rm iso}(\theta)\propto[1+(\theta/\theta_c)^2]^{-a/2}$. The power-law index $a$ reflects the amount of mixing between the shocked jet and confining medium, which depends on the jet’s inital magnetization. Weakly magnetized jets undergo significant mixing, leading to shallow ($a\lesssim2$) angular profiles. Here 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 radial profile, $n_{\rm{}ext}\propto{}R^{-k}$. 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 ($\theta_c\approx0.021$) and a shallow angular structure ($a\approx0.8$) expanding into a stellar wind ($k\approx2$). In addition, the fraction of shock-heated electrons forming a relativistic power-law energy distribution is limited to $\xi_e\approx10^{-2}$ in both shocks.
R. Gill and J. Granot
Fri, 28 Apr 23
26/68
Comments: 5 pages, 3 figures. Submitted