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, θc. We fit model and observed afterglow light curves with a smoothly-broken power-law function with a free-parameter κ that describes the sharpness of the break. We find that the jet break is sharper (κ 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 ≤ κ ≤ 4.6, whereas profiles with a smooth-edged core have a narrower range of 0.1 ≤ κ ≤ 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 θc, whereas for smooth-edged jets, κ is more strongly dependent on the size of θc. 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 κ, 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. © 2021 The Author(s).
GPL and JJF thank Shiho Kobayashi for useful discussions and comments on a pre-submission version. GPL thanks Lekshmi Resmi for valuable discussions. GPL is supported by the Science Technology and Facilities Council (STFC) via grant ST/S000453/1. DAK acknowledges support from Spanish National Research Project RTI2018-098104-J-I00 (GRBPhot), and thanks S. Schulze and M. Blažek for calculations support. IM is a recipient of the Australian Research Council Future Fellowship FT190100574.
With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.