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We present 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). 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 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). The magnetic field is assumed to be tangled with length scale much less than the scale of flow structure. Doppler boost and frequency shift are incorporated via the transfer coefficients. Time delays are ignored, in a first approximation, because even though the instantaneous flow speed is high, the structure of the flow changes at barely relativistic speed.
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. A perturbed, overdense and overpressure jet has a knotty density distribution, but the total intensity map exhibits little structure. We conclude that the appearance of VLBI knots is determined primarily by the Doppler boosting of part of a more extended flow. We discuss a combination of factors likely to limit the number of knots visible at one time on a VLBI map.
This work was supported by NSF grant AST 9120224 and by the Ohio Supercomputer Center from a Cray Research Software Development Grant.
