A Comprehensive Analysis of Textbook-version Afterglow Light Curves of Gamma-Ray Bursts and Implication for Universal Radiation Physics of Baryonic Jets

Abstract

Abstract The standard external shock model in the thin-shell scenario predicts an onset bump in the early optical afterglow light curves of gamma-ray bursts (GRBs). We collect such a textbook-version light-curve sample of 30 GRBs and derive the jet properties from our joint fit to their X-ray and optical afterglow light curves. It is found that the distributions of the isotropic initial Lorentz factors (Γ0), the deceleration radii (R dec), and the magnetic field strength (B 0) are log-normal, but the distributions of the isotropic kinetic energy (E k,iso), medium density (n 0), and magnetization parameter (σ B ≡ ϵ B /ϵ e ) are tentatively bimodal. A tight R dec–B 0–σ B relation is found. It infers a universal ϵ e E k,iso among bursts, plausibly supporting the previous argument of a universal GRB radiation energy among GRBs. A jet break is required for modeling the light curves of 26 GRBs. The distributions of the jet opening angles and the jet-corrected kinetic energies log-normally center at log θ j , c / rad = − 1.51 (standard deviation σ = 0.27) and log ( E k , j , c / erg ) = 51.78 (σ = 0.54), respectively. Those GRBs (19 GRBs), whose prompt gamma-ray emission is well estimated with broad energy-band observations, satisfy the previously discovered L γ,p,iso–E p,z–Γ0 relation, and their gamma-ray radiation efficiencies log-normally distribute in the range from 0.04% to 10% with a central value of 0.42%. Such a low efficiency favors the baryonic fireball model, and the distribution of their baryon mass loading in the GRB ejecta log-normally centers at log ( M fb , c / M ☉ ) = − 5 (σ = 0.75).