  Nuclear and Coulomb collisions in GRB jets create a hot electron-positron
plasma. This collisional heating starts when the jet is still opaque and
extends to the transparent region. The e+- 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 ~ -2.5. This suggests that collisional heating may be the main
mechanism for GRB emission.
