Temporal evolution of electron-positron plasmas

Recently it has become increasingly clear that e-e+ pairs can play an important role in various high-energy astrophysical phenomena and there has been growing interest in understanding these plasmas. In spherical accretion flows and in accretion discs the pairs created close to the Schwarzschild radius have a strong influence on the overall dynamics of the flow, and the spectrum of the emitted high energy radiation shows the typical signature of the pair component. In γ-ray bursters, where the physics of the primary energy release is not fully understood the observed radiation reflects the temporal evolution of the trans-relativistic pair plasma. Zdziarski and Ramaty and Meszaros performed Monte Carlo calculations to obtain the annihilation radiation spectra from relativistic plasmas. Gould and Lightman determined the equilibrium properties and relaxation rates of e-e+ pair plasmas qualitatively and Svensson was able to give analytical expressions for the differential cross sections for annihilation rate and emissivity in isotropic plasmas, leaving only the integrations over the distribution functions of the particles. Nearly all these papers dealt with plasmas having (thermal) Maxwellian distribution functions. It is known from classical plasma physics that this assumption allows to obtain fairly good estimates of the relevant plasma parameters. Nevertheless, for a detailed description of a non-equilibrium plasma one has to solve the transport equation for the system. In particular, in situations where the dynamics of the whole system depends on the actual form of the particle distribution functions (e.g. in accretion flows), a better knowledge of the temporal evolution of pair plasmas is required. 5 references, 1 figure