GRB jet structure and the jet break

We investigate the shape of jet break in within-beam gamma-ray burst (GRB) optical afterglows for various lateral structure profiles. consider cases with and without spreading a range inclinations within core half-opening angle, $\theta_c$. fit model observed afterglow lightcurves smoothly-broken power-law function free-parameter $\kappa$ that describes sharpness break. find is sharper ($\kappa$ greater) when included than absence spreading. For profiles sharp-edged core, parameter has broad $0.1\lesssim\kappa\lesssim4.6$, whereas smooth-edged have narrower $0.1\lesssim\kappa\lesssim2.2$ models both are included. jets, depends strongly on inclination system $\theta_c$, more dependent size Using sample 20 GRBs we nine candidate structures eight structures, while remaining three consistent either. The break, as measured by $\kappa$, can be used an initial check presence where well-sampled at around jet-break time.