Parsec-Scale Synchrotron Emission from Hydrodynamic Relativistic Jets in Active Galactic Nuclei

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Abstract
We present the first numerical simulations of the parsec-scale synchrotron emission from hydrodynamic relativistic jets. The jet structure is calculated using a relativistic time-dependent hydrodynamic code based on an approximate Riemann solver. The radio emission from the model jets is calculated by integrating the transfer equations of the synchrotron radiation, accounting for the appropriate opacity and relativistic effects, such as Doppler boosting and relativistic aberration. In order to study the influence of the external medium, we present two hydrodynamical jet simulations: one with a constant external pressure, and a second model with a decreasing external pressure. Multifrequency radio images of the synchrotron emission from these flows are presented, showing the existence of stationary quasi-periodic knots associated with internal oblique shocks. Whereas for the model with constant external pressure the knots remain almost constant in intensity and even in spacing, the model with decreasing external pressure shows stationary knots of progressively lower intensity and wider spacing as a function of distance down the jet.