2014MNRAS.442..419B

We analyse gamma-ray bursts (GRBs) detected by Fermi/Gamma-Ray Burst Monitor and having single pulse. We fit the light curves with a model having exponential rise and decay parts. We perform a detailed time-resolved spectroscopy using four models: Band, blackbody with a power law (BBPL), multicolour blackbody with a power law (mBBPL) and two blackbodies with a power law (2BBPL). We find that models other than the BBPL give better χ_red^2 for the `hard-to-soft' (HTS) pulses, while for the `intensity tracking' (IT) pulses, the BBPL model is statistically as good as the other models. Interestingly, the energy at the peak of the spectrum resulting from the BBPL model ( ∼ 3kT) is always lower than that of the νFν spectrum of the Band function. The values of the low-energy photon index (α) of the Band function are often higher than the fundamental single particle synchrotron limit, especially for the HTS pulses. Specifically we find two extreme cases - for GRB 110817A (HTS GRB) α is always higher, while for GRB 100528A (IT GRB) α is always within the synchrotron regime. The power-law component of the BBPL model always starts with a delay compared to the blackbody component, and it lingers at the later part of the prompt emission phase. For three HTS GRBs, namely, GRB 081224, GRB 100707A and GRB 110721A this behaviour is particularly significant and interestingly there are reported Large Area Telescope detections for them. Finally, we argue that various evidences hint that neither BBPL nor Band model is acceptable, while 2BBPL and mBBPL are the most acceptable models for the set of GRBs we have analysed.