Annu. Rev. Astron. Astrophys. 1997. 35: 445-502
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6. VARIABILITY OF BLAZARS

6.1. Overview and Relativistic Beaming

Blazars exhibit the most rapid and the largest amplitude variations of all AGN (Stein et al 1976, Angel & Stockman 1980). The combination of extreme variability and relatively weak spectral features suggests the continuum is emitted by a relativistic jet close to the line of sight and hence that the observed radiation is strongly amplified by relativistic beaming (Blandford & Rees 1978). Here we take the point of view that all blazars, whether weak-lined like BL Lac objects or strong-lined like flat spectrum radio-loud quasars (FSRQ), contain essentially similar relativistic jets. They may differ in other aspects of nuclear activity; in particular, BL Lac objects may have less luminous accretion disks and BLRs than FSRQ.

Early multiwavelength studies provided the first global support for the idea of bulk relativistic motion in blazars: The observed radio emission was sufficiently luminous and rapidly variable that, assuming it was due to synchrotron radiation, high X-ray fluxes would be expected from Compton upscattering of the synchrotron photons (the so-called synchrotron self-Compton process), unless the radio emission was relativistically beamed (Hoyle et al 1966, Jones et al 1974a, b).

In addition, many blazars exhibited large-amplitude extremely fast X-ray variations (e.g. Morini et al 1986a, Feigelson et al 1986) that for isotropic emission would violate the limits on Delta L / Delta t for Eddington-limited accretion (Fabian 1979), implying that relativistic effects are important (see also Bassani et al 1983). Finally, direct evidence for relativistic bulk motion has been obtained with VLBI observations of many blazars, which show that apparent superluminal motion is the rule (Mutel 1990, Vermeulen & Cohen 1994).

Here we summarize the data in wavebands that are particularly relevant to understanding the overall continuum (Sections 6.2 - 6.6) and discuss multiwavelength studies, emphasizing the newest results (Section 7). Models are discussed in Section 8.

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