Time-resolved Spectral Properties of Fermi-GBM Bright Long Gamma-Ray Bursts

Abstract

The prompt emission mechanism of gamma-ray bursts (GRBs) is still unclear, and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes. We performed a detailed time-resolved spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray Burst Monitor (GBM). A total of 1490 spectra were obtained and their properties were studied using a typical Band-shape model. Firstly, the parameter distribution of the time-resolved spectrum given as follows: the low-energy spectral index $\alpha \sim -0.72$, high-energy spectral index $\beta \sim -2.42$, the peak energy $E_{\mathrm{p}}\sim 221.69\mathrm{keV}$, and the energy flux $F\sim 7.49\times 10^{-6}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$. More than 80\% of the bursts exhibit the hardest low-energy spectral index ${\alpha }_{\max }$ exceeding the synchrotron limit (-2/3). Secondly, the evolution patterns of $\alpha$ and $E_{\mathrm{p}}$ were statistically analyzed. The results show that for multi-pulse GRBs the intensity-tracking pattern is more common than the hard-to-soft pattern in the evolution of both $E_{\mathrm{p}}$ and $\alpha$. The hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial pulse of multi-pulse GRBs. Finally, we found a significant positive correlation between $F$ and $E_{\mathrm{p}}$, with half of the samples exhibiting a positive correlation between $F$ and $\alpha$. We discussed the spectral evolution of different radiation models. The diversity of spectral evolution patterns indicates that there may be more than one radiation mechanism occurring in the gamma-ray burst radiation process, including photospheric radiation and synchrotron radiation. However, it may also involve only one radiation mechanism, but more complicated physical details need to be considered.

19 pages, 7 figures, 4 tables, accepted and published in RAA

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Cited by in corpus (1)

• Single-Pulse Gamma-Ray Bursts have Prevalent Hard-to-Soft Spectral Evolution