Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Axis-Symmetric Flows

We introduce a simple and economical but effective method for including relativistic electron transport in multi-dimensional simulations of radio galaxies. The method is designed to follow explicitly diffusive acceleration at shocks, and, in smooth flows 2nd order Fermi acceleration plus adiabatic and synchrotron cooling. We are able to follow both the spatial and energy distributions of the electrons, so that direct synchrotron emission properties can be modeled in time-dependent flows for the first time. Here we present first results in the form of some axis-symmetric MHD simulations of Mach 20 light jet flows. These show clearly the importance of nonsteady terminal shocks that develop in such flows even when the jet inflow is steady. As a result of this and other consequences of the fundamentally driven character of jets, we find complex patterns of emissivities and synchrotron spectra, including steep spectral gradients in hot spots, islands of distinct spectra electrons within the lobes and spectral gradients coming from the dynamical histories of a given flow element rather than from synchrotron aging of the embedded electrons. In addition, spectral aging in the lobes tends to proceed more slowly than one would estimate from regions of high emissivity.