Annu. Rev. Astron. Astrophys. 1997. 35: 607-36
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Despite the observational and theoretical advances in the study of compact radio sources, the physical nature of the VLBI components observed in parsec-scale radio jets and their connection to the central engine remains unclear (e.g. Marscher 1996). Turbulent plasma inhomogeneities and shock waves have been proposed with considerable success in explaining specific source properties (Blandford & Königl 1979, Reynolds 1982). In the relativistic jet model, the optically thick cores in the VLBI images represent the base of the jet (located near the apex of the jet cone), and the superluminal features are regions of enhanced emission moving along the jet. Theoretical predictions of synchrotron self-Compton emission from the VLBI components can be compared with observed X-ray fluxes or limits, and an observed excess of predicted versus observed X rays is once again interpreted as evidence for bulk relativistic motion (see Marscher 1987). The corresponding Doppler factors constrain allowed ranges of model parameters. In a core-jet source, the inhomogeneous jet model for the core (Königl 1981) can be combined with (perhaps simplistic) homogeneous spheres for the moving components to estimate the basic properties of the parsec-scale emission regions and to derive Doppler factors for the relativistic motion (Marscher 1987, Zensus et al 1995)

Ghisellini et al (1993) derive Doppler factors for about 100 sources with known VLBI structures by comparing predicted and observed X-ray flux in the synchrotron self-Compton model. The main results agree with those from other beaming indicators (superluminal motion and core to extended-flux density ratio) and support a simple kinematic model of ballistic motion of knots in relativistic jets. The derived Doppler factors are largest for core-dominated quasars, intermediate for BL Lacertae objects, and smallest for lobe-dominated radio galaxies and quasars. For the subsample of 39 superluminal sources, apparent expansion speeds and Doppler factors correlate and have similar average numerical values. This is taken as evidence that the bulk motion causing the beaming also causes the superluminal expansion and that it does not require different pattern and bulk velocities. The corresponding Lorentz factors are about 10 on average, with no significant differences between core- and lobe-dominated quasars and BL Lacs. The derived viewing angles differ, with radio galaxies, lobe-dominated quasars, BL Lacertae objects, and core-dominated quasars representing increasingly aligned objects.

In the case of 3C 345, the synchrotron self-Compton model reconciles well the component radio spectra, including the flat radio spectrum of the core, with the observed flux density, spectrum, and correlated variability in the X rays (Zensus et al 1995, Unwin et al 1994, Unwin et al 1997). The best-fit model requires a very small opening angle (~ 0.5°) for the portion of the jet near the apex of the cone that represents the flat-spectrum radio core; the apparent opening angle is much larger owing to projection effects. The inverse Compton calculations suggest that typically the core and its nearest moving component dominate the X-ray emission. The model is consistent with a heavy proton/electron jet that would yield enough power to fuel the outer radio lobe, and it argues for a small angle to the line of sight of about 1°. This can be relaxed if a pattern speed smaller than the true fluid speed is allowed. Combination of the superluminal speed (from VLBI) and the Doppler factor deduced from the synchrotron self-Compton calculation for the more recently ejected component (C7) implies that the jet bends away from the line of sight (from theta ltapprox 2° to ltapprox 10°), and such combination also implies that it accelerates from gamma ltapprox 5 to ltapprox 10 over the range of (deprojected) distance from the nucleus of ltapprox 3 to ltapprox 20 h-1 pc (Unwin et al 1997).

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