2025ApJ...987..124D

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/Gamma-Ray Burst Monitor. 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. In 67% of the GRBs, the thermal component gradually decreases 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% and 77.8% evolve in a similar fashion in terms of the low-energy spectral index α and the peak energy E_p, respectively. Of the GRBs 50.0% and 72.2% exhibit comparable correlations for flux-α and α-E_p, respectively. For flux-E_p both the main and second bursts show a positive correlation. Moreover, from the perspective of the temporal evolution of characteristic radii, the transition from the main to the second burst appears 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 with it.