Exploring Physically-Motivated Models to Fit Gamma-Ray Burst Spectra

We explore fitting gamma-ray burst spectra with three physically-motivated models, and thus revisit the viability of synchrotron radiation as the primary source of GRB prompt emission. We pick a sample of 100 bright GRBs observed by the Fermi Gamma-ray Burst Monitor (GBM), based on their energy flux values. In addition to the standard empirical spectral models used in previous GBM spectroscopy catalogs, we also consider three physically-motivated models; (a) a Thermal Synchrotron model, (b) a Band model with a High-energy Cutoff, and (c) a Smoothly Broken Power Law (SBPL) model with a Multiplicative Broken Power Law (MBPL). We then adopt the Bayesian information criterion (BIC) to compare the fits obtained and choose the best model. We find that 42% of the GRBs from the fluence spectra and 23% of GRBs from the peak-flux spectra have one of the three physically-motivated models as their preferred one. From the peak-flux spectral fits, we find that the low-energy index distributions from the empirical model fits for long GRBs peak around the synchrotron value of -2/3, while the two low-energy indices from the SBPL+MBPL fits of long GRBs peak close to the -2/3 and -3/2 values expected for a synchrotron spectrum below and above the cooling frequency.