Nuclear and Coulomb collisions in GRB jets create a hot e ^ { \pm } plasma .
This collisional heating starts when the jet is still opaque and extends to the transparent region .
The e ^ { \pm } plasma radiates its energy .
As a result , a large fraction of the jet energy is converted to escaping radiation with a well-defined spectrum .
The process is simulated in detail using the known rates of collisions and accurate calculations of radiative transfer in the expanding jet .
The result reproduces the spectra of observed GRBs that typically peak near 1 MeV and extend to much higher energies with a photon index \beta \sim - 2.5 .
This suggests that collisional heating may be the main mechanism for GRB emission .