Modelling of the non-thermal emission from inhomogeneous jets in active galactic nuclei

Abstract

Some active galactic nuclei (AGNs) are strong sources of non-thermal radiation. It is expected that this radiation is produced in collimated outflows of plasma moving with relativistic speeds, the so-called jets. The non-thermal radiation from AGN jets extends over the entire electromagnetic spectrum, i.e. from the radio band to the very high energy gamma-rays. This radiation is strongly variable in every energy range. The repro- duction of these properties of radiation with theoretical models allows us to understand processes and conditions occurring in AGN jets. The AGN jets have been the subject of extensive research for several decades. However, some fundamental questions still remain unanswered. The study of the AGN jets can give the answers to such basic questions as: How are the non-thermal particles accelerated inside the AGN jets? Where are the high energy γ-rays produced? What is the type of particles responsible for the non-thermal radiation? Most commonly, the non-thermal radiation from AGN jets is modeled with one- zone homogeneous lepton model. In such a model, the emission region is approximated by a spherical blob. This blob moves along the jet with relativistic speed. Such model assumes that the conditions inside the blob are constant and homogeneous. The evident advantage of this model is the fact that it is determined by only a few free parameters. However, the observed spectral details and the complex temporal behaviors require more sophisticated models. For example, the one-zone synchrotron self-Compton model is not able to explain the persistent emission from AGN jets. Some AGNs are observed in a low activity state lasting for months. This persistent emission is observed also in the very high energy gamma- ray range. In such a long period of time, the parameters in the moving emission region should change significantly. In this context, I propose the stationary and inhomogeneous jet model. The parameters in the model change with the distance from the jet base. In contrast to the one-zone model, I approximate the emission region with a parsec- scale cone which better describes the shape of AGN jets measured with the radio telescopes. Due to the elongated shape of the jet, I take into account the non-locally produced photons when calculating the inverse Compton process. The equilibrium state of particles in the jet is obtained by the dedicated generation method. My approach provides a unique tool for modeling of the jets in the persistent, low-activity emission state. 1The results of the commonly used emission models of AGN jets are inconsistent with the unification model of AGNs. The unification model assumes that the radio galaxies are counterparts of blazars observed at large angles. Whereas, the modeling of radio galaxies requires different parameters than the modeling of blazar type of AGNs. I show that the spectra of both, radio galaxies and blazars, considered in this thesis can be obtained in terms of the unified model. For this purpose, I develop the two-component jet model, in which fast-moving plasma, close to the jet axis, is surrounded by slower plasma. In contrast to previous models of this type, I take into account the strong interrelation between different jet components in the calculations of the equilibrium spectrum of relativistic electrons. In the third model, I investigate the consequences of the production of gamma-rays in the vicinity of a super-massive black hole in blazars. Some models, supported by the observations of the extremely fast flares, assume that the very high gamma-rays are produced relatively close to the black hole. In the case of blazars, the jet propagates towards the observer. Hence, these gamma-rays have to pass the jet radiation field before they escape to the observer. These gamma-rays can be strongly absorbed initiating the inverse Compton electro-positron pair cascades. I explore this idea in terms of the observed excess in the hard X-rays in nearby blazar Mrk 421.