Numerical Simulations of Blazar Jets and their Non-thermal Radiation

In the past years relativistic (magneto)hydrodynamic simulations have been used extensively to study the time-dependent hydrodynamic properties of extra galactic jets. While these simulations have been very successful in studying the formation, collimation, propagation and termination of relativistic jets, the models used to compute synthetic images from the hydrodynamic properties were relatively simple. On the other hand, there exist several theoretical models which assume a very simple hydrodynamic evolution, but treat the non-thermal particles and their emitted radiation with great detail. It was the aim of this work to include a detailed treatment of the non-thermal particles and their synchrotron radiation in high-resolution shock-capturing relativistic hydrodynamic (RHD) simulations. To achieve this goal we have developed a transport scheme for the non-thermal particles by treating them as "tracer" fluids in the RHD equations. Their temporal evolution is calculated using an analytic kinetic equation solver, and their synchrotron radiation is computed in a time-dependent manner taking into account the relevant relativistic effects, (e.g., light travel times to the observer). The energy density of a dynamically negligible magnetic field is assumed to be a fraction of the energy density of the thermal fluid. Two models have been developed for the parameterization of the acceleration of non-thermal particles at relativistic shocks: A type-E model where only the strength of the shock influences the number of accelerated particles and a type-N model where the shock strength only influences their energy distribution. We have demonstrated that our numerical method is able to capture the essentials of the temporal and spatial evolution of the non-thermal particles and the observed synchrotron radiation with a reasonable accuracy when applied to subparsec scale relativistic jets. Understanding the physical processes connected to the observed X-ray blazar light curves has been the main object of research !

with our new numerical tool. For the first time, the hydrodynamic evolution and the synchrotron radiation of a blazar jet was simulated consistently. We have simulated collisions of density inhomogeneities (shells) within a blazar jet. The results have shown that the efficiency of the observed synchrotron radiation varies with the relative velocities of the shells as well as with the amount of initially available mass. The surrounding medium plays an important role, because it heats up the shells prior to the collision, a fact which is neglected in simpler models. Assuming that the observed radiation results from the interaction of shells within a blazar jet, we have developed an analytic model which enables the determination of the unobservable parameters of the jets (i.e., length and velocity of the shells) from the light curve. The parameters predicted by the model have been compared to results of our simulations and we find that the agreement is surprisingly good, given the simplicity of the model. In addition, several long-term simulations of collisions of many shells have shown that a model of an intermittently working central engine seems to produce light curves more similar the observed ones than a model in which the central engine ejects a continuous outflow.