Particle acceleration in a flow accreting through shock waves

It was shown recently (Donner, 1979; Sawada et al., 1986; Spruit, 1987) that accretion with angular momentum onto a compact object is possible by a process in which the dissipation and angular momentum transport are due to a system of stationary shock waves. The accreting mass spirals many times through the shocks before reaching the central object. In this kind of shocked flow diffusive particle acceleration works well even for low shock strengths, in contrast to the case of an isolated shock. The process is illustrated with analytical examples for diffusion of relativistic particles in a periodically shocked flow, and with a numerical solution for diffusion in a detailed two-dimensional flow model. Flat particle spectra (spectral index around 2) are found under rather mild assumptions on the diffusion coefficient and the probability for escape of particles from the accreting flow. It is proposed that this process is involved in the production of energetic particles in active galactic nuclei.