A Study of the Spectral Properties of Gamma-Ray Bursts with the Main and Second Bursts

TL;DR

This study analyzes the spectral properties of main and second gamma-ray bursts, revealing similarities in their spectral evolution and suggesting a shared physical origin, with implications for understanding GRB jet composition.

Contribution

It provides a detailed Bayesian spectral comparison of main and second GRB bursts, demonstrating their spectral and physical continuity, which was not previously established.

Findings

83.3% of bursts contain a thermal component

Main and second bursts show similar spectral evolution patterns

Both bursts follow the same Amati and Yonetoku relations

Abstract

The origins of the main burst and second burst of gamma-ray bursts (GRBs) and the composition of their jets remain uncertain. To explore this complex subject more thoroughly, we conduct a spectral analysis on 18 GRBs with a main and a second burst observed by Fermi/GBM. First, we employ Bayesian time-resolved spectral analysis to compare the spectral components of the main and the second burst, finding that $83.3\%$ of the main and second bursts contain a thermal component. $67\%$ of the GRBs, the thermal component gradually decreased from the main to the second burst and the number of spectra exceeding the "Synchrotron line-of-death" is significantly higher in the main burst than in the second burst. Subsequently, we ascertain that for both the main and second bursts, $71.4\%$ of the low-energy spectral index $\alpha$ and $77.8\%$ of the peak energy $E_{p}$ evolve in a similar fashion. There are $50.0\%$ and $72.2\%$ of the GRBs exhibit comparable correlations for the $Flux-\alpha$ and $\alpha-E_{p}$, respectively. For $Flux-E_{p}$ both the main and second burst show a positive correlation. Moreover, from the perspective of the temporal evolution of characteristic radii, the transition from the main to the second burst appeared to be seamless. Finally, we find that both the main and the second burst follow the same Amati relation and Yonetoku relation. Our analysis strongly indicates that the second burst is a continuation of the main burst and is highly likely to share a common physical origin.