From the observational point of view, a simplified classification of the AGN population can be based just on two parameters1: the radio loudness and the width of the emission lines. Following different definitions, a radio loud object has a radio flux at 5 GHz at least 10 times larger than the optical-B one or has a radio luminosity larger than 1033erg s−1Hz−1 (Stocke et al. 1992). These objects represent only the 10–15% of the entire AGN population, which is dominated by radio quiet sources.
Depending on the presence or absence of broad emission lines, AGN are usually classified as type 1 or type 2, respectively, in analogy with the early Seyfert 1 – Seyfert 2 distinction.
The dust torus
Observations of Seyfert galaxies have revealed the presence of intermediate objects, between type 1 and type 2, that show only weakly detectable broad line components or important variations of the line width. Therefore, it has been proposed that both AGN types have the same physical nature and that the differences are due to varying amounts of absorbing material along the line of sight. This led
1Narrow Line Seyferts 1 (NLS1) galaxies seem to escape this simple classification. In fact, NLS1 are characterised by strong narrow lines and small widths of the broad Balmer lines (<2000 km s−1), like in Seyfert 2, but their strong optical and ultraviolet FeII emission and their ratio of optical to X-ray luminosity identify them rather as Seyfert 1. Many of the characteristics of NLS1 are understood as a consequence of their super-Eddington accretion rates (Collin & Kawaguchi 2004).
Figure 2.1: Schematic view of an AGN, where the main components are indicated (Urry & Padovani 1995).
The unified models propose that all these components (apart maybe for the jet) are present in all AGN and that the different AGN types correspond to different orientations of the AGN with respect to the observer.
to the introduction of a dust shell in the standard AGN picture, having any possible geometry, from a 4πshell with varying thickness to an angle-dependent axisymmetric structure or a patchy distribution of dust. The idea of a torus-like structure was motivated by the discovery by Antonucci & Miller (1985) of broad lines in the Seyfert 2 galaxy NGC 1068 when observed in linearly polarized light.
They argued that the broad lines in the spectrum are produced by reflection of radiation from the nuclear region. Historically, this study (afterwards supported by the observation of ionisation cones in a number of Seyferts; e.g. Wilson et al. 1993) was fundamental for the formulation of the unified model of AGN that considers the observational properties of the different AGN classes as a result of an orientation effect between the observer and the torus (Antonucci 1993). In this scheme, AGN have a dusty torus extending from 1 to 100 pc from the centre and type 1 AGN are believed to be objects of which the observer can view the inner regions within the torus opening angle (Fig. 2.1). In these objects both the BLR and NLR are visible. On the other hand, when the line of sight crosses the torus, the BLR appears to be hidden and this results in type 2 AGN spectra. This unification, first proposed for Seyfert galaxies, was then extended to include the observational differences of type 1 and type 2 objects also among radio loud AGN, with the radio galaxies being considered as obscured radio loud quasars.
Peculiar objects questioning the unified model
The unification model for type 1 and 2 objects is supported by the observations indicating that in general type 2 AGN have stronger absorbed X-ray spectra, a redder optical continuum and stronger
The unified models 11 spectral features from the host galaxy (Wilkes 2004) than type 1. On the other hand, the structure of the absorbing material seems to be more complex than a simple dust torus. In fact, a few sources have shown significant variations of the X-ray spectral shape, in some cases consistent with a temporary switching off of the nuclear source (e.g. NGC 4051, Matt et al. 2003) or with a clumpy torus where the absorption can strongly vary on relatively short time scales of days (e.g. NGC 1365, Risaliti et al.
2007). In addition some Seyfert 1 show strong intrinsic X-ray absorption (e.g. ESO 323-G077 with NH = 6.6×1022cm−2; Malizia et al. 2007), while some Seyfert 2 do not show intrinsic absorption.
Among the latter, NGC 3147 has been found to have an X-ray spectrum typical of Seyfert 1, even though it is optically classified as type 2. Bianchi et al. (2008) have proposed that the BLR of this object is not just hidden, but completely missing, maybe due to the relatively small accretion rate of this AGN, which might be insufficient for the BLR outflow to form. This indicates that at least for this object the key parameter to describe the observational characteristics is not the orientation but an intrinsic property (as for instance the mass accretion rate) that prevents the formation of the BLR.
The unification of radio loud AGN
When a jet is observable, the axisymmetric structure of the standard AGN model implies a large variety of the observed properties (and so classifications) as a function of the observing angle. A unification of the radio loud AGN has therefore been proposed based on the direction of the jet axis with respect to the line of sight. In this scenario, it is believed that radio loud AGN in which the jet is observed end-on belong to the blazar class, whereas ordinary radio loud quasars are viewed at larger angles from the jet axis and radio galaxies have jets seen in the plane of the sky. The particular jet orientation in blazars is responsible for the high luminosities (up to 1048−49erg s−1, when isotropic luminosity is assumed) and fast variability observed in these objects and for the apparent superluminal motions in the jet, all properties that can be explained as due to plasma moving at relativistic speeds towards the observer along the line of sight.
The blazar sequence
The spectral energy distributions (SED) of radio loud AGN present in general two main broad peaks (inνFν representation), one in the IR to X-ray range and the second one in the MeV to GeV band, depending on the AGN type (Figs. 2.6). These two peaks are often interpreted as due to the same population of relativistic electrons, accelerated in the jet, emitting synchrotron radiation at lower energies and scattering some seed photons through inverse Compton processes at higher energies. In the synchrotron self-Compton models (SSC), the seed photons for the inverse Compton process are assumed to be those emitted by synchrotron, whereas in the external Compton models (EC) the seed photons come from a source external to the jet, as for example the accretion disc (directly or first reprocessed by the ambient medium), the broad line region or the dusty torus (see von Montigny et al.
1995 for a review).
Fossati et al. (1998) have carried out a study on the SEDs of a sample of blazars and proposed a unification of the different blazar subclasses based on a continuum of properties that seems to be determined only by the blazar luminosity. The three blazar subclasses considered are the high-frequency-peaked BL Lac (HBL), the low-high-frequency-peaked BL Lac (LBL) and the flat-spectrum radio quasars (FSRQ), ordered by increasing luminosities. Fossati et al. (1998) noticed that the fre-quency of the two SED peaks decreases with increasing luminosity, the distance between the two peaks being about constant (left panel in Fig. 2.2). In addition, the ratio between the inverse
Comp-Figure 2.2:Left:The average SED of different blazar subclasses as found by Fossati et al. (1998). The points represent the data and the lines are analytical approximations. Going from the lower to the upper curves, HBL, LBL and FSRQ SEDs are shown. Right: Blazar SEDs for different values of black hole mass and accretion rate, as predicted by the model of Ghisellini & Tavecchio (2008). For FSRQs (upper thick lines), the accretion rate was assumed constant (L/LEdd=0.3) andMvaried as indicated. For BL Lac (lower thin lines), a mass of M=109M⊙was chosen and the accretion rate (i.e.L/LEdd) was varied as labelled.
ton and the synchrotron peak fluxes increases with increasing luminosity. Ghisellini et al. (1998) explained this sequence as due to an increasing importance of the external radiation field going from HBL to FSRQs. In HBL the electron cooling would be less efficient and dominated by synchrotron processes, determining the extension of the spectrum to higher energies and a low contribution to the inverse Compton. The weakness of the external photon field in BL Lac objects is confirmed by the quasi-absence of emission lines. On the other hand, in FSRQs an intense radiation field external to the jet would cool down faster the electrons (shifting the SED peaks to lower energies) and mainly through inverse Compton radiation.
The discovery of blazars with broad lines but a HBL-type SED (e.g. Padovani et al. 2002, Maraschi et al. 2008) and of extremely luminous HBL (e.g. H1517+656; Beckmann et al. 1999), and the in-creasing evidence that LBL are more numerous than HBL (which would contradict blazar luminosity functions if LBL are more luminous than HBL; Padovani 2007) have questioned over the years the validity of this blazar sequence. Recently, retrieving some ideas already proposed by Maraschi &
Tavecchio (2003), Ghisellini & Tavecchio (2008) have elaborated a new approach to the problem and proposed that the SED properties and jet power in blazars can be represented by two parameters, the black hole mass M and the accretion rate ˙M (together determining the disc luminosity, Ldisc). Assum-ing that the jet power scales with ˙M, that the BLR exists only above a critical value of Ldiscand that its distance from the black hole scales as L1/2disc, and that the location in the jet where most of the emission is produced scales with M, Ghisellini & Tavecchio (2008) explain the blazar sequence as follows. In FSRQs, the large black hole mass and high accretion rate determine the presence of powerful disc and jet and position the BLR at large distances from the centre, where the BLR-jet interaction gives origin to Compton-dominated blazars (right panel in Fig. 2.2). With evolution, the accretion rate decreases and therefore the disc radiates less efficiently and the BLR becomes smaller. As a consequence, in some cases the most active region of the jet can still interact with the BLR and in others it is farther
The X-ray emission from AGN 13 out with respect to the BLR radiation field. This would explain the existence of broad-line blazars with HBL-type SED. When ˙M decreases below a critical value (Ldisc/LEdd = 3×10−3), the BLR is too weak and too small2to interact with the jet and this represents the situation in BL Lac objects.