Quasi-blackbody component in the prompt emission of gamma-ray bursts

The characteristic sub-MeV peak in the energy spectrum of the prompt GRB emission can be attributed to blackbody emission of the photosphere of the outflow, having a temperature of approximately 10(9) K. An additional non-thermal spectral component is then attributed to additional dissipation of the kinetic energy in the outflow. Here we study this interpretation and analyze instantantaneous spectra in the 20- 2000 keV range and show that such a two-component model can be fit to most spectra well. Interestingly, the thermal component exhibits a recurring behaviour over emission pulse structures. Both the temperature and the energy flux vary as broken power-laws. During the pre-break phase the temperature is approximately constant while the energy flux rises. Furthermore, the ratio of the observed thermal flux to the emergent flux increases as a power law over the whole pulse. It is argued that these observations hold the key to our understanding of the prompt emission and of the properties of the site from which it emanates.