  We discuss the prompt emission of GRBs, allowing for $\gamma\gamma$ pair
production and synchrotron self-absorption. The observed hard spectra suggest
heavy pair-loading in GRBs. The re-emission of the generated pairs results in
the energy transmission from high-energy gamma-rays to long-wavelength
radiation. Due to strong self-absorption, the synchrotron radiation by pairs is
in 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.
