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

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 (Γ _0 ), 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 $\mathrm{log}{\theta }_{{\rm{j}},{\rm{c}}}/\mathrm{rad}=-1.51$ (standard deviation σ = 0.27) and $\mathrm{log}({E}_{{\rm{k}},{\rm{j}},{\rm{c}}}/\mathrm{erg})=51.78$ ( 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 –Γ _0 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 $\mathrm{log}({M}_{\mathrm{fb},{\rm{c}}}/{M}_{\odot })=-5$ ( σ = 0.75).

Keywords

• Gamma-ray bursts