International Astronomy and Astrophysics Research Journal

Aims: We have investigated the properties of thermal and non-thermal components as well as the effect of the thermal components on the overall gamma-ray burst (GRB) spectra with 20 GRBs observed by Fermi/GBM.

Methodology: We first comprehensively compare the fitting results of the two different models (Band Only and Band+Blackbody (BB)) in the time-resolved spectra. We then use a power-law with an exponential cutoff + Blackbody+ power law (CPL+BB+PL) model to fit the time-resolved spectra. Finally, we present the spectral evolutions between the spectral parameters and their physical significance.

Results: The thermal component decreases the low-energy index α and does not seem to affect high-energy index β; the thermal component makes the peak energy E_p increase and it leads to faster decay of E_p in decaying episodes. Four GRBs can be fitted by CPL+BB+PL model, which shows that PL component appears in the energy spectrum at the beginning of the fireball explosion.

Conclusion: (i) The radiation mechanisms of a few bursts are consistent with the synchrotron radiation while three bursts can be explained by jitter radiation; (ii) comparing the particle acceleration models in GRBs, the magnetic reconnection is more suitable to the prompt emission than the internal shock; (iii) the correlation between spectral peak energy E_p and thermal temperature kT shows that there are magnetically-dominant jets in the GRBs; (IV) the E_p−F_p (energy flux) relation is weaker in Band+BB, which may be regarded as one of the evidences of the existence of the thermal radiation in GRB prompt emission.