GRB jet structure and the jet break

Gavin P Lamb,Ilya Mandel,Nial R Tanvir,Joseph John Fernández,Andrew J Levan,D Alexander Kann

doi:10.1093/mnras/stab2071

Abstract

ABSTRACT 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.

Highlights

The achromatic breaks observed in the light curves of some gammaray burst (GRB) afterglows have been used to argue that these sources are jet-like in nature (Rhoads 1997)
Each panel shows the data for the named GRB, the red curves show a random sample of 100 parameter sets for equation (6) from the Markov Chain Monte Carlo (MCMC) fit to the data
We have shown that for an observer at a line-of-sight inclination angle ι within the core opening angle θ c of a GRB, the temporal evolution of the afterglow light curve around the jet-break time is distinct between core-dominated jets with a sharp-edged core and those with a smooth-edged core profile

Summary

Introduction

The achromatic breaks observed in the light curves of some gammaray burst (GRB) afterglows have been used to argue that these sources are jet-like in nature (Rhoads 1997). The observed time of the afterglow jet break after the prompt GRB contains information about the angular size, θ j of the outflows that produce these transients The temporal index α of the light curve before a jet break, and the steeper decline α2 after it, provide information about the ambient medium density profile and the spectral regime of the emission (Granot & Sari 2002) and constrain energy injection into the outflow (Zhang et al 2006). Where lateral spreading is included, the post-break decline index is α ≤ −p, where p is the electron energy spectral slope Where lateral spreading is included, the post-break decline index is α ≤ −p, where p is the electron energy spectral slope (e.g. Rhoads 1999; Sari et al 1999; Zhang & MacFadyen 2009)

Methods

Results

Discussion

Conclusion