Modeling the emission of active galactic nuclei at Fermi's era

Active galactic nuclei (AGN) are the most energetic objects known in the universe. Their fantastic luminosity is due to efficient conversion of gravitational energy of mass accreted on super-massive black-holes at the center of galaxy. About 10% of AGN are even more incredible as they display relativistic jets on galaxy scales. Those jets are observed at all energies, from far radio to the highest gamma-rays. Despite intense study since their discovery in the 50's and more and more observations, encouraged by rapid progress in instrumentation, AGN are still widely misunderstood. The questions of formation, composition, and acceleration of jets are central but still a matter of debates. Models aiming at reproducing observed emission have been developed throughout the years. The most common one, the one-zone model, often relies on ad-hoc hypothesis and does not provide a satisfactory answer. The two-flow paradigm developed at IPAG and based on an original idea from Sol et al (1989) aims at giving a more coherent and physical representation of AGN jets. The principal assumption is that jets are actually composed of two coaxial flows: an inner spine made of a pure pair plasma, moving at relativistic speed and responsible for the non-thermal observed emission surrounded by an external sheath, made of a baryonic MHD plasma, mildly relativistic but carrying most of the power. The two-flow paradigm finds roots in observations as well as theoretical arguments and has been able to explain many AGN features. During my PhD, I studied this paradigm and contributed to the development of a numerical model based on its concepts. I have been particularly interested in the inverse Compton scattering of thermal photons, fundamental process in the modeling of AGN emission, as well as the Compton rocket effect, key to the acceleration of the spine in the two-flow paradigm. However, taking completely into account the inverse Compton emission can be very time consuming. To accomplish fast and efficient computation of the external Compton emission, I have had to formulate new analytical approximations of the scattering of a thermal distribution of photons. I have also studied the Compton rocket effect, responsible for the acceleration of the inner spine in the two-flow paradigm. I showed that the resulting bulk Lorentz factor of the flow in the complex photon field of an AGN is subject to variations along the jet as a function of the distance to the central engine. These variations can have drastic effects on the observed emission and could induce variability, both spatially and temporally. I also showed that the terminal bulk Lorentz factor obtained are compatible with physical conditions expected in jets and with observations. The complete model produce spectral energy distribution (SED) comparable to observed ones. However, the model is by nature erratic and it is difficult to make a direct link between the model parameters (input) and the SED (output). Unfortunately, standard data fitting procedures (e.g. based on gradient methods) are not adapted to the model due to its important number of parameters, its important computing time and its non-linearity. In order to circumvent this issue, I have developed a fitting tool based on genetic algorithms. The application of this algorithm allowed me to successfully fit several SED. In particular, I have also showed that the model, because based on a structured jet model, can reproduce observations with low bulk Lorentz factor, thus giving hope to match observations and theoretical requirements in this matter.