Prompt Ultraviolet-to-Soft X-Ray Emission of Gamma-Ray Bursts: Application to GRB 031203?

We discuss the prompt emission of gamma-ray bursts (GRBs), allowing for γγ pair production and synchrotron self-absorption. The observed hard spectra suggest heavy pair loading in GRBs. The reemission of the generated pairs results in the energy transmission from high-energy gamma rays to long-wavelength radiation. Because of strong self-absorption, the synchrotron radiation by pairs is in an optically thick regime, showing a thermal-like spectral bump in the extreme-ultraviolet/soft X-ray band, other than the peak from the main burst. Recently, the prompt soft X-ray emission of GRB 031203 was detected thanks to the discovery of a delayed dust echo, and it seems to be consistent with the model prediction of a double-peak structure. The confirmation of the thermal-like feature and the double-peak structure by observation would indicate that the dominant radiation mechanism in GRBs is synchrotron rather than inverse Compton radiation.