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 $\alpha$, the high-energy power-law index $\beta$, the peak energy in the observed $\nu F_{\nu }$ spectrum $E_{\text{p}}$, and the difference between the low- and high-energy power-law indices $\Delta s=\alpha -\beta$. Using the distributions of $\Delta s$ and $\beta$, 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 $\beta$, 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

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• The Bright and the Slow -- GRBs 100724B \& 160509A with high-energy cutoffs at $\lesssim 100$MeV
• The sharpness of gamma-ray burst prompt emission spectra
• A comprehensive statistical study on gamma-ray bursts
• Bayesian Time-Resolved Spectroscopy of Multi-Pulsed GRBs: Variations of Emission Properties amongst Pulses
• Clustering of gamma-ray burst types in the Fermi-GBM catalogue: indications of photosphere and synchrotron emissions during the prompt phase
• Hydrodynamic properties of gamma-ray bursts outflows deduced from thermal component
• A New Measurement of the Spectral Lag of Gamma-Ray Bursts and its Implications for Spectral Evolution Behaviors
• Synchrotron emission in GRBs observed by Fermi: Its limitations and the role of the photosphere
• Emission from accelerating jets in gamma-ray bursts: Radiation dominated flows with increasing mass outflow rates
• On the $\alpha$-Intensity Correlation in Gamma-Ray Bursts: Subphotospheric Heating with Varying Entropy
• On Spectral Peak Energy of Swift Gamma-Ray Bursts
• Time-Reversed Gamma-Ray Burst Light Curve Characteristics as Transitions between Subluminal and Superluminal Motion
• Constrains on the physics of the prompt emission from a distant and energetic gamma-ray burst GRB 220101A
• Study of prompt emission of short gamma ray bursts using multi-color blackbody: a clue to the viewing angle
• Statistical properties of Fermi GBM GRBs' spectra
• Photons' scattering in a relativistic plasma with velocity shear: generation of high energy power-law spectra
• The Spectral Sharpness Angle of Gamma-ray Bursts
• GRB Spectra in the complex of synchrotron and Compton processes