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

4.2. Spectral Properties

Until recently, most monitoring studies were confined to images of the total intensity of a given source in one or more particular observing bands. This suffices to determine basic morphology and component kinematics, but it lacks important physical information that can only be obtained from measuring the jet spectra from multifrequency work (e.g. Walker et al 1996) and the magnetic field distributions from polarization imaging (e.g. Roberts et al 1991, Cotton 1993, Leppänen et al 1995, Kemball et al 1996).

For 3C 345, by combining images taken at different frequencies at quasisimultaneous epochs (within about six months from each other), spectral information can be obtained and used to measure the basic parameters of the component synchrotron spectra: the turnover flux density and frequency and the integrated flux in the range 4-25 GHz (Lobanov & Zensus 1997). The observed luminosity variations suggest a variable pattern Lorentz factor in at least one component. The core turnover frequency and integrated flux show significantly weaker variations compared to the moving components, which suggests that the primary emission mechanism in the jet might differ from that of the core (although blending of the core with a new component can confuse the situation); at least two components, C4 and C5, show a peaked evolution of turnover frequency. Such spectral properties are important for testing the hypothesis that the jet components are caused by relativistic shocks (Rabaça & Zensus 1994, Marscher & Gear 1985, Hughes et al 1989b, Marscher et al 1992, Gómez et al 1993). However, the evidence in favor of the shock model is inconclusive so far. The total flux density variations in some sources have been adequately explained with shocks (Hughes et al 1989b). In 3C 345, strong shocks with a variable Doppler factor may well be the mechanism at work in the jet at least near the core, although at larger distances (> 2 milliarcsec), the shock model alone does not suffice to explain the observed properties, e.g. the required intrinsic accelerations and long-component life times (Lobanov & Zensus 1997). Alternative models have been proposed that involve interaction with the ambient plasma (Rose et al 1987), e.g. the two-fluid model (Sol et al 1989, Pelletier & Roland 1990, Pelletier & Sol 1992) or nonsynchrotron emission mechanisms, i.e. bremsstrahlung (Weatherall & Benford 1991). For the scenario of an induced helical geometry of the jet, the two-fluid model predicts jet kinematics and flux density variability of a form not unlike those seen for at least one component (C7) of 3C 345 (Roland et al 1994).

The shock-in-jet hypothesis has been applied with success in a number of other sources. Marcaide et al (1994a) apply a detailed model of shock components moving on twisted trajectories to the quasar 4C39.25. BL Lac is perhaps the best-understood case for the interpretation of VLBI components by shocks (Mutel et al 1990).