Annu. Rev. Astron. Astrophys. 1997. 35: 445-502 Copyright © 1997 by Annual Reviews. All rights reserved

8.2. The Relativistic Jet: Synchrotron Radiation from the Outer Regions

Self absorption is important in blazars even at high radio frequencies. The flat radio spectra likely derive from inhomogeneity along the jet - that is, from contiguous jet regions with different self-absorption cutoffs (Blandford & Königl 1979, Marscher 1980, Köonigl 1981). Often, discrete spectral components can be recognized, sometimes related to individual VLBI components or to the unresolved core. The spectral evolution of these components - in particular, the shift of the self-absorption cutoff to longer wavelengths and concurrent changes in polarization angle - can be well modeled as enhancements in the local radio emission due to shocks propagating along a relativistic jet whose particle density and magnetic field decrease outward (Marscher & Gear 1985, Hughes et al 1989, 1994, Litchfield et al 1995, Stevens et al 1996). The ability of such models to account at the same time for both total flux and polarization variability is encouraging, although the derived angle of view tends to be larger than for simple unification schemes (Kollgaard 1994, Urry & Padovani 1995).

The high polarization of the optical emission and the spectral continuity from the radio (albeit with increasing spectral index) imply that synchrotron radiation is the dominant emission mechanism in most blazars up to the UV range and in some cases up to the X rays. It is not yet clear, however, whether the relativistic electrons responsible for the high-frequency synchrotron emission are simply the high energy tail of those emitting at radio wavelengths or represent a distinct population in a different part of the jet, presumably closer to the central engine. Radio through optical light curves show some correlation, but the lags have not been quantified even for the best-observed sources (Stevens et al 1994, Tornikoski et al 1994). Uncorrelated flares are also present (Hufnagel & Bregman 1992). An understanding of the radio-optical connection is also hampered by the far-IR observational gap (now accessible with ISO). The self-absorption turnover of the optical synchrotron component cannot be determined unambiguously, and therefore its physical parameters are poorly constrained.