Summary:

Electron-positron pair production is an important cooling mechanism for plasmas at mildly relativistic temperatures. A thorough understanding of the process is necessary to explain the hard X-ray spectra of AGNs and gamma-ray bursters. We investigate thermal plasmas with temperatures kT m_ec² and optical depths 1 <  < 5, including pair processes, Comptonization and bremsstrahlung. Results are presented for equilibrium and impulsively heated models. We find that, in the former case, the observed spectrum is featureless, but in the latter case it can show a broad, flat annihilation feature. We then discuss non-thermal pair production in plasmas confined by strong magnetic fields, such as those thought to exist at the surface of neutron stars. We show that two photon pair production cannot give an annihilation feature, but that magnetic pair production ( B e⁺e^- ) can give a significant annihilation line. Finally we consider simple dynamical systems and the effect of expansion on the spectrum of a gas in which pair production is important. Adiabatic cooling steepens the flat spectral feature to ^-3 in the thermal case, and in the non-thermal case the spectrum has a flat component with a turnover at ~ 2 MeV.