Time-resolved spectral study of Fermi gamma-ray bursts having single pulses

Abstract

We analyze gamma-ray bursts (GRBs) detected by \textit{Fermi}/Gamma-Ray Burst Monitor (GBM) 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 $\chi_{red}^2$ 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 ($\sim3kT$), is always lower than that of the $\nu F_{\nu}$ spectrum of the Band function. The values of the low energy photon index ($\alpha$) 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) $\alpha$ is always higher, while for GRB~100528A (IT GRB) $\alpha$ is always within the synchrotron regime. The PL component of the BBPL model always starts with a delay compared to the BB 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 LAT 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 analyzed.