Afterglow light curves from misaligned structured jets
Abstract
GRB 170817A/GW 170817 is the first gammaray burst (GRB) clearly viewed far from the GRB jet's symmetry axis. Its afterglow was densely monitored over a wide range of frequencies and times. It has been modelled extensively, primarily numerically, and although this endeavour was very fruitful, many of the underlying model parameters remain undetermined. We provide analytic modelling of GRB afterglows observed offaxis, considering jets with a narrow core (of halfopening angle θ_{c}) and powerlaw wings in energy per unit solid angle (ɛ = ɛ_{c}Θ^{a} where Θ = [1 + (θ/θ_{c})^{2}]^{1/2}) and initial specific kinetic energy (Γ_{0}  1 = [Γ_{c, 0}  1]Θ^{b}), as well as briefly discuss Gaussian jets. Our study reveals qualitatively different types of light curves that can be viewed in future offaxis GRBs, with either single or double peaks, depending on the jet structure and the viewing angle. Considering the lightcurve shape rather than the absolute normalizations of times and/or fluxes, removes the dependence of the light curve on many of the highly degenerate burst parameters. This study can be easily used to determine the underlying jet structure, significantly reduce the effective parameter space for numerical fitting attempts and provide physical insights. As an illustration, we show that for GRB 170817A, there is a strong correlation between the allowed values of Γ_{c, 0} and b, leading to a narrow strip of allowed solutions in the Γ_{c, 0}b plane above some minimal values Γ_{c, 0} ≳ 40, b ≳ 1.2. Furthermore, the Lorentz factor of the material dominating the early light curve can be constrained by three independent techniques to be Γ_{0}(θ_{min, 0}) ≈ 57.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 April 2020
 DOI:
 10.1093/mnras/staa538
 arXiv:
 arXiv:2001.02239
 Bibcode:
 2020MNRAS.493.3521B
 Keywords:

 radiation mechanisms: general;
 gammaray burst: general;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 14 pages, 17 figures. Published in MNRAS