This thesis investigates the physics of relativistic outflows in astrophysical scenarios. We focus on collimated flows, known as jets, that occur in Active Galactic Nuclei (AGN) and Gamma-Ray Bursts (GRBs).
The production and early propagation of relativistic jets is examined in the case of acceleration by dynamically dominant tangled magnetic fields. It is shown that such a configuration behaves in a similar fashion as a pure particle pressure dominated jet, however, it can avoid radiative losses and inverse Compton drag, which hamper particle dominated flows. The radiative signatures of such jets and several complications, such as dissipation of magnetic energy and radiation drag, are considered.
To investigate the energetics and propagation of the kpc scale jet, we study the jet in the nearby galaxy M87. We find that the complex spectral behavior and the surprising correlation between radio brightness and the optical spectral index can be explained by simple adiabatic acceleration of the radiating particles. The lack of significant spectral evolution is consistent with magnetic fields below equipartition by a factor of a few.
The impact of the enormous energy flux in the AGN jets on the large scale intergalactic environment is studied on the basis of a simple dynamical model. In general, the flow will displace the surrounding medium, leading to a depression in the X-ray surface brightness. We show how a grid of models can be used to infer important physical parameters, such as the average kinetic jet power from Chandra observations of radio galaxies embedded in clusters.
Relativistic flows also occur in Gamma-Ray Bursts. It has generally been accepted that only models in which the gamma rays originate from internal variations in the flow (internal shocks) can explain the complex temporal signatures of GRBs. We present a new model, based on external dissipation of kinetic energy stored in dense bullets, that can also explain the millisecond variability seen in some bursts. The basic observational characteristics of such a model are presented, along with a preliminary analysis of the requirements that the viability of such a flow imposes on the central engines of bursts.