Throughout the years, the observed phenomenological (e.g. bolometric luminosities, mass of the SMBH) and spectral (e.g. emission lines of the optical spectra) properties of AGN have been catalogued and used as parameters for classifying and understanding their nature and physics.
Today, under the current AGN paradigm, it is widely accepted that the central en- gine of an AGN comprises a SMBH-accretion disc system that powers the AGN system and the relativistic jet (1). Observations at optical and ultraviolet (UV) wavelengths of the nuclear activity are often obscured either by the torus or warped gas and dust well outside the accretion disc and broad line regions, when the source is viewed at particular angles (e.g. the central engine is obscured by the torus when viewed edge-on, see Figure 1.1).
The Current AGN Paradigm - The Standard AGN Model
It is generally accepted that while AGN are divided into separate classes, all AGN are es- sentially made up of the same components. These include a central engine which powers
the system by the accretion of surrounding matter from an accretion disc, a relativistic jet which transports energy from the central system out into the galactic and extragalac- tic environment, broad and narrow line regions of gas clouds surrounding the central engine including the jet, and an optically-thick dust torus which surrounds the accretion disc, extending up to 100 pc from the central SMBH. The following sections describe these components in greater depth.
1.1.1 The Central Engine
One of the key components when dealing with AGN is its energy generation mechanism (i.e. the central engine) which drives and sustains the AGN system. In the early 1960s, Hoyle & Fowler (248) suggested that the launch of the relativistic jet might be possible with the gravitational contraction of matter around a stellar-like nucleus with up to 108
solar masses. This mechanism would be able to release gravitational potential energy which may be stored in toroidal magnetic field lines during the accretion process by conservation of angular momentum.
Development of this scenario of energy generation by Zel’dovich & Novikov in 1965 (288) suggested that the production of jets had to be coupled with the processes of a black hole (> 106 solar masses) in the form of the potential energy found in the gravitational field of the black hole and the rotation of the black hole itself in order to generate the energy output observed from these radio sources. In 1969, Lynden-Bell (274) further developed the idea that the accretion of matter onto a rotating compact object (such as matter accretion onto a rotating Kerr SMBH) provides the deep gravitational potential well necessary to launch and sustain the gyroscopic stability of these jets over Mpc scales. The dynamical interaction between the SMBH and the accretion disc which contin- ually feeds the SMBH forms an important aspect of understanding the features of the central engine. The accretion disc-SMBH system and its dynamics are highly complex and have been the subject of extensive research. A review of accretion disc physics can be found in e.g. Melia (282). Matter in-falling into the central nucleus loses angular momen- tum through viscous and turbulent processes. The velocities of matter in the accretion disc are dependent on its radius from the nuclear region. Matter in the inner regions of the accretion disc will have higher velocities compared to matter moving in the outer regions of the disc. This leads to energy loss via friction which subsequently causes ther-
Figure 1.1: This diagram shows the active galactic nuclei (AGN) classification scheme. This classification scheme is based on observational, phenomenological and spectral properties including the orientation of the AGN with respect to the observer, such as the jet aligned blazar class. The green arrows indicate the viewing angle which is used to determine the various AGN sub-classes. The image is based on the unified AGN model by Urry & Padovani (290).
mal radiation at IR, optical and UV wavelengths. The loss of kinetic energy in the inner regions of the accretion disc also enables matter to be accreted onto the SMBH as an- gular momentum is transported out of the accretion disc. Another important aspect of the accretion disc system is that it radiates at different wavelengths, both thermally and non-thermally. This radiation produces seed photons which are re-radiated by particles in other regions of the central nucleus (e.g. broad and narrow line region gas clouds). Photons produced from emission in the accretion disc also feature as seed photons in jet emission models. These will be addressed in Chapter 2.
1.1.2 The Relativistic Jet
The relativistic jet is the collimated outflow of energetic particles occurring along the poles of the disc or torus (Figure 1.1). The jet comprises relativistic plasma carrying matter and energy to locations remote from the central engine. In the current AGN paradigm, jets are thought to originate in the vicinity of the SMBH and are powered by the SMBH-accretion disc system. The physical processes which lead to the formation and collimation of the jet are still the subject of great debate. The primary questions pertain to the mechanisms through which plasma is accelerated to relativistic speeds and collimated into jets with opening angles as small as few degrees (277).
The physical geometry, composition and emission processes which occur within the jet and as part of the jet phenomena is a subject in and of itself. A comprehensive look into various aspects of the relativistic jet system can be found in Boettcher et al. 2012 (156). In this section, a brief account of the jet is given. Relativistic jets have extended conical structures, launched to Mpc scales, outward from the central engine. Jets ap- pear to be made of compact components (typically referred to knots) which move down- stream at apparent superluminal speeds > 25c (where c is the speed of light) (212; 221). Relativistic jets also show curvature within the compact region and between the parsec- scale and kiloparsec-scale jet (277). In 2001, Jorstad et al. (212) found that 43% of their 42
γ-ray bright AGN sources exhibited jet bending angles of > 20◦ at parsec scales. These were noted to be consistent with amplification by projection effects of modest actual changes in the position angle component (212).
The studies of blazars often concern the structures and dynamics of jets. This is be- cause blazars are categorised based on the close alignment of their jets to the line-of-
sight. This causes the blazar emission spectrum to be dominated by the non-thermal emission of the jet rather than thermal emission from gas or dust either from the accre- tion disc, torus or the broad and narrow line region gas clouds surrounding the jet. Due to this, modelling the structure, magnetic fields, and jet plasma forms an intrinsic aspect of blazar emission models. The current relativistic jet paradigm, including its parameters and structure will be discussed in further detail in Section 2.5.
The ubiquitous nature of relativistic jets as part of the AGN phenomenon provides a wealth of information on the central engine. Observations of the jet at different wave- lengths allow researchers to probe the activity of the central engine by monitoring fea- tures such as flux variability timescales, spectral indices and radio morphologies.
An important aspect which arises when studying the physics and physical properties of relativistic jets is the jet composition. The elements within the jet produce the neces- sary environments which result in the broadband emission observed from these sources. This jet composition is primarily important when modelling emission models (these are discussed in Chapter 2).
1.1.3 The Torus
The torus is a geometrically symmetrical toroidal structure comprising of gas and dust surrounding the accretion disc, at distances between 30 pc and 100 pc from the central engine (110) (see Figure 1.1). The torus is optically thick and reprocesses a fraction of UV radiation from the inner regions of the AGN (i.e. accretion disc) and emits this repro- cessed radiation at infrared (IR) wavelengths. In the unified AGN paradigm, this dust torus is responsible for obscuring emission from the central regions including the broad line region gas clouds when AGN are viewed edge-on. The notion of obscuring mate- rial surrounding the nucleus was initially used to explain the anisotropic distribution of emission features (i.e. the absence of broad high-ionisation lines in the spectra in Type 2 AGN) (290).
1.1.4 The Broad and Narrow Line Region Gas Clouds
In the vicinities above and below the accretion disc, gas ’clouds’ are heated by radiation from the accretion disc. These regions produce emission lines which can be divided into two types: (i) the broad line region (BLR) produces broad emission lines and, (ii)
the narrow line region (NLR) produces narrow emission lines. It was suggested that the difference in the widths of the permitted emission lines may be a result of the differential Doppler shifts due to motions from individual clouds within the BLR and NLR regions (265).
As shown in Figure 1.1, the BLR clouds are located closer to the central engine at distances of ≲ 1 pc (286). This proximity ensures that the BLR clouds are hot (with temperatures derived from thermal motion alone reaching ∼ 109 K) and denser with
electron densities reaching up to Ne ∼ 109 cm−3moving at velocities between 1000 km
s−1 and 25 000 km s−1 (298). Due to the high temperatures and its proximity to the central engine, the permitted emission lines from the BLR are very luminous and the BLR are constantly fed by the UV radiation of the nuclear region.
The NLR is located further from the central engine than the BLR, reaching distances of up to 1 kpc (286). Gas clouds in the NLR have lower temperatures (≈ 15000 K) (289) and velocities (v≲ 500 km s−1) (298). The electron density of this regions is also consid- erably lower (Ne∼ 102- 106cm−3) compared to the BLR (298).
The BLR and NLR clouds are particularly important in the study of blazars as these regions interact with the relativistic jet in several different ways. In blazar emission mod- els (see Section 2.6), seed photons from the BLR and NLR clouds are inverse Compton scattered to higher energies producing the high-energy component observed in blazar spectral energy distributions. It is also possible that these ’clouds’ move into regions of the jet, increasing particle densities in that particular region, (or a precessing jet moves into matter rich BLR or NLR regions) causing particle interaction with matter outside the jet.