Time development of relativistic thermal plasmas

Time development of an impulsively heated plasma, first studied by Guibert and Stepney (1985; AAA 39.062.002), is investigated numerically for plane-parallel and spherical geometries. For an initial proton temperature of 0.03 - 0.1 mpc2 and a pair-free optical thickness to the Thomson scattering of order unity, electron-positron pairs are produced on the time scale of about 10 times the initial Thomson time and the optical thickness becomes 5 - 20 times the initial thickness at the maximum, roughly proportional to the initial proton temperature. The subsequent evolution is governed by the photon transfer, since the energy density is dominated by high-energy photons at this stage. Pairs annihilate and the plasma cools on the time scale of about 10 times the initial Thomson time. The photon spectrum is of Comptonized bremsstrahlung and a prominent Wien peak is formed without the feature of a high-energy tail claimed by Guilbert and Stepney. Otherwise our results are similar to theirs for typical cases. The dependence on the initial conditions and geometries is also discussed. (author)