1.2 Unified Schemes

All this might seem complicated, but in recent years we have developed a consistent scenario which at least explains the Type 0 / 1 / 2 distinction. We have in fact come to understand that some classes of apparently different (and therefore classified under different names) AGN might actually be intrinsically the same class of objects seen at different angles with the line of sight (see for example Antonucci 1993 and Urry and Padovani 1995 and references therein).

Figure 1. A schematic (and highly idealized) diagram of the current paradigm for radio-loud AGN (not to scale). Surrounding the central black hole is a luminous accretion disk. Broad emission lines are produced in clouds (dark spots) orbiting above the disk and perhaps by the disk itself. A thick dusty torus (or warped disk) obscures the broad-line region from transverse lines of sight; some continuum and broad-line emission can be scattered into those lines of sight by hot electrons (black dots) that pervade the region. A hot corona above the accretion disk may also play a role in producing the hard X-ray continuum. Narrow lines are produced in clouds (grey spots) much farther from the central source. Radio jets, shown here as the diffuse jets characteristic of low-luminosity, or FR I-type, radio sources, emanate from the region near the black hole, initially at relativistic speeds (Urry and Padovani 1995; copyright Astronomical Society of the Pacific, reproduced with permission).

The main idea, based on various observations and summarized in Fig. 1, is that emission in the inner parts of AGN is highly anisotropic. The current paradigm for AGN includes a central engine, possibly a massive black hole, surrounded by an accretion disk and by fast-moving clouds, probably under the influence of the strong gravitational field, emitting Doppler-broadened lines. More distant clouds emit narrower lines. Absorbing material in some flattened configuration (usually idealized as a toroidal shape) obscures the central parts, so that for transverse lines of sight only the narrow-line emitting clouds are seen (Type 2 AGN), whereas the near-infrared to soft-X-ray nuclear continuum and broad-lines are visible only when viewed face-on (Type 1 AGN). In radio-loud objects we have the additional presence of a relativistic jet, roughly perpendicular to the disk, which produces strong anisotropy and amplification of the continuum emission (``relativistic beaming''), which I discuss in more detail in Sect. 2.1. For reasons still unclear, BL Lac objects have extremely weak emission lines, and their continuum is very strong and non-thermal (i.e., due to synchrotron and, at high energies, inverse Compton emission or perhaps hadronic processes).

This axisymmetric model of AGN implies widely different observational properties (and therefore classifications) at different aspect angles. Hence the need for ``Unified Schemes'' which look at intrinsic, isotropic properties, to unify fundamentally identical (but apparently different) classes of AGN. Seyfert 2 galaxies have therefore been ``unified'' with Seyfert 1 galaxies, whilst low-luminosity (FR I) and high-luminosity (FR II) radio galaxies have been unified with BL Lacs and radio quasars respectively (see Antonucci 1993 and Urry and Padovani 1995 and references therein). In other words, BL Lacs are thought to be FR I radio galaxies with their jets at relatively small ( 20-30°) angles w.r.t. the line of sight. Similarly, we believe FSRQ to be FR II radio galaxies oriented at small ( 15°) angles, while SSRQ should be at angles in between those of FSRQ and FR II's (15 40°). Blazars are then a special class of AGN which we think have their jets practically oriented towards the observer.

In general, different AGN components are important at different wavelengths. Namely: 1. the jet emits non-thermal radiation, via electromagnetic (synchrotron and inverse Compton) and perhaps hadronic processes, all the way from the radio to the -ray band (Mastichiadis 1998; Dar 1998); 2. the accretion disk probably emits thermal radiation, peaked in optical/ultraviolet/soft-X-ray band; 3. the obscuring material (torus) will emit predominantly in the infrared. These different components are apparent, for example, in the multifrequency spectrum of 3C 273 (Lichti et al. 1995) the first quasar to be discovered and one of the best studied.

At this point one might ask: what has all this to do with -ray emission? The answer in the next section.