Annu. Rev. Astron. Astrophys. 1997. 35: 607-36 Copyright © 1997 by Annual Reviews. All rights reserved

3.4. Broadband Activity and Emergence of New Jet Components

The study of multiwavelength total flux-density variability offers an important diagnostic in testing models for the "inner jet" in blazars, and it is complementary to direct observation of structural variability (see Marscher & Bloom 1994, Marscher 1995, Marscher 1996). In particular, nonthermal flaring activity from radio to -ray frequencies is expected to show frequency-dependent time delays that are model specific. The -ray emission detected in a large number of blazars (Von Montigny et al 1995) is most likely to be produced from inverse-Compton scattering from relativistic jet electrons, although the details of this process are not yet clear. Several such blazars are therefore being subjected to multiwavelength monitoring campaigns that are beginning to show model-discriminating results (see Marscher 1996). The superluminal quasar 3C 279 (e.g. Carrara et al 1993) has been intensively monitored over a large spectral range (Maraschi et al 1994, Grandi et al 1996), and there is VLBI component activity near coincident with -ray flaring (Wehrle et al 1994a, b). For the blazar PKS 0420-014, helical structure and new component ejection were predicted from simultaneous optical and -ray flaring (Wagner et al 1995), and so far VLBI imaging has confirmed curvature in the structure of this source. Several other -ray active blazars, including 0528+134, 3C 273, and 0836+710, are also being investigated with VLBI at millimeter wavelengths, and here new features have been observed near -ray flares and at the onset of millimeter-flux density activity, which perhaps suggests a causal connection (Krichbaum et al 1995, Britzen et al 1997). A correlation between -ray outbursts and superluminal motion might suggest the existence of relativistic -ray jets, i.e. within as close as a few light-days to the central engine in such objects. One of several alternative explanations is that the -ray bursts might be related to precessing relativistic e^± beams (Roland et al 1994).

BL Lac is perhaps the best example for a correlation between variability in the centimeter regime and the appearance of superluminal features that can be modeled with shocks (Mutel et al 1990, Mutel et al 1994, Denn & Mutel 1996). In this source, the position angles of the axes of the curved trajectories of subsequent components shift, suggesting evidence for precession caused by a binary black hole system (Mutel et al 1994). Note that such a scenario was also proposed for the superluminal quasar 1928+738 (Hummel et al 1992b, Roos et al 1993).

In 3C 345, several components first appeared following a major flux density increase at centimeter wavelengths, but not all components in this source have been related to such pronounced flux density changes; for example, the recent feature C7 was first seen during a deep minimum in the total flux density. In this source X-ray variability is correlated to the radio variations supporting the synchrotron self-Compton model for this source (Unwin et al 1997). In 3C 273, a correlation appears to exist between the ejection of superluminal features with flares in the radio and infrared/optical regimes (Krichbaum et al 1990a, Abraham et al 1996). A possible correlation of the speed of a component in this source with the amplitude of a perhaps related optical outburst has been speculated to exist (Bhabadzhanyants & Belokon 1992). In 3C 454.3, a new component appeared around 1983, with an apparent mean motion of 0.81 ± 0.04 mas yr^-1 (v / c = 20.4 h^-1 ± 1.0). Extrapolation gives a time of zero separation of 1982 ± 3, i.e. close to the time of a radio outburst (Pauliny-Toth et al, in preparation).