GRB jet structure and the jet break

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 light curves 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. For profiles with a sharp-edged core, the sharpness parameter has a broad range of 0.1 less than or similar to kappa less than or similar to 4.6, whereas profiles with a smooth-edged core have a narrower range of 0.1 less than or similar to kappa less than or similar to 2.2 when models both with and without lateral spreading are included. For sharp-edged jets, the jet break sharpness depends strongly on the inclination of the system within theta(c), whereas for smooth-edged jets, kappa is more strongly dependent on the size of theta(c). Using a sample of 20 GRBs, we find 9 candidate smooth-edged jet structures and 8 candidate sharp-edged jet structures, while the remaining 3 are consistent with either. The shape of the jet break, as measured by the sharpness parameter kappa, can be used as an initial check for the presence of lateral structure in within-beam GRBs where the afterglow is well-sampled at and around the jet-break time.

Automated summary

The study investigates the shape of the jet break in within-beam gamma-ray burst optical afterglows for various lateral jet structure profiles, finding that the sharpness of the jet break is higher when lateral spreading is included. The sharpness of the jet break is strongly dependent on the inclination angle and the size of the core for different jet profile shapes.