Synchrotron Cooling in Energetic Gamma-Ray Bursts Observed by the Fermi Gamma-Ray Burst Monitor

Abstract

We study the time-resolved spectra of eight GRBs observed by Fermi GBM in its first five years of mission, with 1 keV - 1 MeV fluence $f>1.0\times 10^{-4}$ erg cm${}^{-2}$ and signal-to-noise level $\text{S/N}\ge 10.0$ above 900 keV. We aim to constrain in detail the spectral properties of GRB prompt emission on a time-resolved basis and to discuss the theoretical implications of the fitting results in the context of various prompt emission models. We perform time-resolved spectral analysis using a variable temporal binning technique according to optimal S/N criteria, resulting in a total of 299 time-resolved spectra. We fit the Band function to all spectra and obtain the distributions for the low-energy power-law index $\hat{I}\pm$, the high-energy power-law index ${\hat{I}}^2$, the peak energy in the observed $\hat{I}½F_{\hat{I}}½$ spectrum $E_{\text{p}}$, and the difference between the low- and high-energy power-law indices $\hat{I}”s=\hat{I}\pm -{\hat{I}}^2$. Using the distributions of $\hat{I}”s$ and ${\hat{I}}^2$, the electron population index $p$ is found to be consistent with the "moderately fast" scenario which fast- and slow-cooling scenarios cannot be distinguished. We also apply a physically motivated synchrotron model, which is a triple power-law with constrained power-law indices and a blackbody component, to test for consistency with a synchrotron origin for the prompt emission and obtain the distributions for the two break energies $E_{\text{b,1}}$ and $E_{\text{b,2}}$, the middle segment power-law index ${\hat{I}}^2$, and the Planck function temperature $kT$. A synchrotron model is found consistent with the majority of time-resolved spectra for these eight energetic Fermi GBM bursts with good high-energy photon statistics, as long as both the cooling and injection break are included and the leftmost spectral slope is lifted either by inclusion of a thermal component or when an evolving magnetic field is accounted for.

20 pages, 7 figures, 8 tables, accepted for publication in A&A