GRB jet structure and the jet break

Understand

We investigate the shape of the jet break in within-beam gamma-ray burst (GRB) optical afterglows for various lateral jet structure profiles.

• We consider cases with and without lateral spreading and a range of inclinations within the jet core half-opening angle, $\theta_c$.
• We fit model and observed afterglow lightcurves with a smoothly-broken power-law function with a free-parameter $\kappa$ that describes the sharpness of the break.
• We find that the jet break is sharper ($\kappa$ is greater) when lateral spreading is included than in the absence of lateral spreading.