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.