Blazars are a special kind of active galaxy characterized by very rapid variability, high and variable polarization, superluminal motion, and very high luminosities - in short they are the most ``active'' kind of AGN. Given that the theme of this COSPAR session is accretion disk phenomenology, I begin by describing the relation of blazars, which in general do not show direct observational evidence of accretion disks, to AGN that do.
Figure 1 illustrates the current AGN paradigm, in which a supermassive black hole provides the central power source. An equatorial accretion disk provides a means for funneling matter onto the black hole. The surrounding emission line clouds, both broad emission line clouds and narrow emission line clouds farther out, are less apparent in the spectra of blazars than in other AGN but both narrow and broad lines are sometimes seen. An optically thick screen, shown in Figure 1 as a thick torus, obscures the central continuum and broad-line emission along some lines of sight. Some fraction of AGN, perhaps 10%, are radio loud, and these have jets which are at least initially relativistic.
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Figure 1: Schematic drawing of the current paradigm for AGN (not
to scale; example lengths in parentheses).
Surrounding the central black hole
(for M = 108 M black hole,
RS  2 GM
/c2 ~ 3 x 1013 cm)
is a luminous accretion disk
(~ 1-30 x 1014 cm).
Broad emission lines are produced in clouds orbiting above the disk
(at ~ 2-20 x 1016 cm) and perhaps by the disk itself.
A thick dusty torus (inner radius ~ 1017 cm;
or warped disk) obscures the
broad-line region (BLR) from transverse lines of sight; some continuum and
broad-line emission can be scattered into those lines of sight by hot
electrons 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 much farther from the central source
(1018-1020 cm).
In radio-loud AGN,
radio jets emanate from the region near the black hole
(extending from 1017 to several times 1024 cm,
a factor of ten larger than the largest galaxies),
initially at relativistic speeds. (From
Urry & Padovani 1995;
copyright Astronomical Society of the Pacific, reproduced with permission.)
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There are strong indications that blazars are the radio-loud AGN seen more or less end-on down the jet, so that the bulk relativistic motion of the emitting plasma causes radiation to be beamed in a forward direction, making the variability appear more rapid and the luminosity appear higher than in the rest frame (Urry & Padovani 1995, and references therein). First, virtually every blazar exhibits superluminal motion in high-resolution radio maps (Vermeulen & Cohen 1994), which is easily explained by relativistic bulk motion along the line-of sight. Second, they are all highly polarized, at least some of the time. Indeed, the classical definition of blazars as optically violently variable AGN turns out to be consistent with defining them by high polarization (excluding radio-quiet AGN polarized by scattering), or by superluminal motion, or most recently, by their strong gamma-ray emission (taking duty cycle into account). Third, blazars are well matched with radio galaxies, which are known to have jets more nearly in the plane of the sky, in terms of number and luminosities, as well as in terms of properties not affected by beaming (host galaxy, environment, and so on). Their radio variability, multiwavelength variability, and polarization characteristics can all be well explained by shocks in an aligned relativistic jet.
Blazars have very high brightness temperatures, based on the rapid variability of their radio and optical emission. This is described in an excellent review article (Wagner & Witzel 1995) so I say nothing more here about intra-day variability; the remainder of this talk concerns multiwavelength properties of blazars, particularly at the higher energies.