Synchrotron cooling in energetic gamma-ray bursts observed by the Fermi Gamma-Ray Burst Monitor

Context. We study the time-resolved spectral properties of energetic gamma-ray bursts (GRBs) with good high-energy photon statistics observed by the Gamma-Ray Burst Monitor (GBM) onboard the Fermi Gamma-Ray Space Telescope.
Aims: 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.
Methods: Our sample comprises eight GRBs observed by the Fermi GBM in its first five years of mission, with 1 keV-1 MeV fluence f> 1.0 × 10^-4 erg cm^-2 and a signal-to-noise ratio level of S/N ≥ 10.0 above 900 keV. We performed a 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 performed Band function fits to all spectra and obtained the distributions for the low-energy power-law index α, the high-energy power-law index β, the peak energy in the observed νF_ν spectrum E_p, and the difference between the low- and high-energy power-law indices Δs = α - β. We also applied a physically motivated synchrotron model, which is a triple power-law with constrained power-law indices and a blackbody component, to test the prompt emission for consistency with a synchrotron origin and obtain the distributions for the two break energies E_b,1 and E_b,2, the middle segment power-law index β, and the Planck function temperature kT.
Results: The Band function parameter distributions are α = -0.73^+0.16_-0.21, β =ي-2.13^+0.28_-0.56, Ep = 374.4^+307.3_-187.7 , keV (log₁₀Ep = 2.57^+0.26_-0.30), and Δs = 1.38^+0.54_-0.31 , with average errors σ_α ~ 0.1, σ_β ~ 0.2, and σ_Ep ~ 0.1E_p. Using the distributions of Δs and β, the electron population index p is found to be consistent with the "moderately fast" scenario, in which fast- and slow-cooling scenarios cannot be distinguished. The physically motivated synchrotron-fitting function parameter distributions are E_b,1 = 129.6^+132.2_-32.4 keV, E_b,2 = 631.4^+582.6_-309.6 keV, β = -1.72^+0.48_-0.25 , and kT = 10.4^+4.9_-3.7 keV, with average errors σ_β ~ 0.2, σ_Eb,1 ~ 0.1E_b,1, σ_Eb,2 ~ 0.4E_b,2, and σ_kT ~ 0.1kT. This synchrotron function requires the synchrotron injection and cooling break (i.e., E_min and E_cool) to be close to each other within a factor of ten, often in addition to a Planck function.
Conclusions: A synchrotron model is found that is consistent with most of the time-resolved spectra for 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 including a thermal component or when an evolving magnetic field is accounted for.

Appendix A is available in electronic form at http://www.aanda.org