Simulations of superluminal radio sources

Abstract

Superluminal motions observed in very long-baseline interferometry (VLBI) radio sources are generally believed to be associated with moving shocks in a collimated relativistic flow. Recent progress in numerical relativistic fluid dynamics now makes it possible to carry out numerical simulations of such flows. In this paper we calculate the flow that occurs when a moving shock caused by source variability encounters a reconfinement shock induced by a pressure gradient in the medium through which the jet propagates. We then use the relativistic radiative transfer equation for an optically thin gas to determine the radio emission from such a flow. These calculations take into account all three of the relevant relativistic effects i.e. the relativistic Doppler effect, aberration and retardation. We find that our simulated images not only contain stationary and superluminal knots, but also reproduce many of the other things that are observed, such as extended features with sharp inner edges and limb brightening.