Simulated VLBI Images From Relativistic Hydrodynamic Jet Models

Abstract

A series of simulated maps showing the appearance in total intensity of flows computed using a recently developed relativistic hydrodynamic code (Duncan \& Hughes 1994: ApJ, 436, L119) are presented. The radiation transfer calculations were performed by assuming 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 $<30^{\circ}$, differential Doppler boosting leads to a series of axial knots of emission, 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 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 ($\sim 5$) exhibit complex intensity maps, while faster jets (Lorentz factor $\sim 10$) 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.