A morphological analysis of gamma-ray burst early optical afterglows

TL;DR

This study classifies early optical afterglow lightcurves of gamma-ray bursts into four morphological types, constrains key model parameters through observations and simulations, and suggests a moderately magnetized jet structure.

Contribution

It introduces a systematic morphological classification of GRB optical afterglows and constrains model parameters, providing insights into jet magnetization and electron energy distribution.

Findings

Four morphological types identified with specific observational ratios.

Favored magnetic equipartition parameter $oldsymbol{ ext{} ext{ extmu}_B^f}$ ranges from 10^{-6} to 10^{-2}.

Electron equipartition parameter $oldsymbol{ ext{ extmu}_e^{r,f}}$ is around 0.01.

Abstract

Within the framework of the external shock model of gamma-ray bursts (GRBs) afterglows, we perform a morphological analysis of the early optical lightcurves to directly constrain model parameters. We define four morphological types, i.e. the reverse shock dominated cases with/without the emergence of the forward shock peak (Type I/ Type II), and the forward shock dominated cases without/with $\nu_m$ crossing the band (Type III/IV). We systematically investigate all the Swift GRBs that have optical detection earlier than 500 s and find 3/63 Type I bursts (4.8%), 12/63 Type II bursts (19.0%), 30/63 Type III bursts (47.6%), 8/63 Type IV bursts (12.7%) and 10/63 Type III/IV bursts (15.9%). We perform Monte Carlo simulations to constrain model parameters in order to reproduce the observations. We find that the favored value of the magnetic equipartition parameter in the forward shock ($\epsilon_B^f$) ranges from $10^{-6}$ to $10^{-2}$, and the reverse-to-forward ratio of $\epsilon_B$ ($R_B$) is about 100. The preferred electron equipartition parameter $\epsilon_e^{r,f}$ value is 0.01, which is smaller than the commonly assumed value, e.g., 0.1. This could mitigate the so- called "efficiency problem" for the internal shock model, if $\epsilon_e$ during the prompt emission phase (in the internal shocks) is large (say, $\sim 0.1$). The preferred $R_B$ value is in agreement with the results in previous works that indicates a moderately magnetized baryonic jet for GRBs.