Simulated VLBI images from relativistic hydrodynamic jet models

A series of simulated maps showing the appearance in total intensity of flows computed using a recently developed relativistic hydrodynamic code (Duncan & Hughes) are presented. The radiation transfer calculations were performed by assuming that the flow is permeated by a magnetic field and fast particle distribution in energy equipartition, with energy density proportional to the hydrodynamic energy density (i.e., pressure). We find that relativistic flows subject to strong perturbations exhibit a density structure consisting of a series of nested bow shocks, and that this structure is evident in the intensity maps for large viewing angles. However, for viewing angles less than 30 degrees, differential Doppler boosting leads to a series of knots of emission that lie along the jet axis, similar to the pattern exhibited by many VLBI sources. The appearance of VLBI knots is determined primarily by the Doppler boosting of parts of a more extended flow. To study the evolution of a perturbed jet, a time series of maps was produced, and an integrated flux density light curve created. The light curve shows features characteristic of a radio-loud AGN: small-amplitude variations and a large outburst. We find that in the absence of perturbations, jets with a modest Lorentz factor (similar to 5) exhibit complex intensity maps, while faster jets (Lorentz factor similar to 10) are largely featureless. We also study the appearance of kiloparsec jet-counterjet pairs by producing simulated maps at relatively large viewing angles; we conclude that observed hot spot emission is more likely to be associated with the Mach disk than with the outer bow shock.