Astron. Astrophys. 344, 61-67 (1999)

2. The VLBI jet of 3C 273

3C 273 is one of the most extensively studied extragalactic sources. Detailed observations have been accumulated during almost 35 years, including simultaneous determinations of its spectrum from radio to -ray energies (Lichti et al. 1995) and multiwavelengths variability data for equally wide energy ranges (von Montigny et al. 1997).

3C 273 has been also subject to intense VLBI monitoring since Cohen et al. (1971) measured its superluminal expansion velocity. The first hybrid maps were obtained by Readhead et al. (1979) at 10.7 GHz, and from them Pearson et al. (1981) clearly established the existence of superluminal motions in this object. Since then at least ten superluminal components, labelled from C1 to C10 according to the identification epoch, have been detected moving away from the core (e.g. Unwin et al. 1985, Biretta et al. 1985, Cohen et al. 1987; Zensus et al. 1988, 1990; Krichbaum et al. 1990, Abraham et al. 1996). Additional short-lived components almost surely have been missed during the gaps between observations. Superluminal motions can be detected up to a projected distance of parsecs from the core (Davis et al. 1991). There has been some discussion about whether the motions are ballistic or curved (e.g Zensus et al. 1988, Krichbaum et al. 1990). Analyzes of all available data (e.g. Abraham et al. 1996) show that the velocities of the individual components do not change with time, but clearly vary from one component to another, ranging from 4.9c to 7.7c . These results seem to support a picture where the motions are ballistic at least up to distances of 15 mas.

We shall work with the hypothesis that the central engine of blazars ejects plasma at relativistic bulk velocities and relatively small viewing angles (e.g. Blandford & Königl 1979). The plasma is collimated into a jet which can be detected at parsec-scale distances from the unresolved core by VLBI techniques at cm and mm-wavelengths. It is usually thought that the VLBI core is the narrow end of the undisturbed jet flow, while the superluminal components are propagating disturbances like shock waves (e.g. Marscher 1996a,b). The term `inner jet' will be used here to denote this unresolved, optically thick, innermost section of the source through which the relativistic plasma flows towards the extended mas-radio jet.

In Table 1 we summarize the main information for the different superluminal features for which high-quality data are available. We list for each component the superluminal velocity in units of c, the position angle , the formation time and the references from which the data were obtained. The superluminal velocities were calculated from the proper motions assuming a Robertson-Walker universe with Hubble constant km s^-1 Mpc^-1 and deceleration parameter . The quoted errors represent the differences in the values quoted by different authors instead of the smaller formal errors obtained by model fitting the VLBI observations or adjusting straight lines to the velocity data. The position angle of the individual components varies between and . The arcsec radio and optical jet is directed towards , imposing an additional constraint to the precessing jet amplitude. We shall use this information to reproduce the kinematic evolution of the inner jet from which the components were expelled.

Table 1. Superluminal components in the VLBI jet in 3C 273.
Notes:
1: Cohen et al. (1983), 2: Biretta et al. (1985), 3: Zensus et al. (1988),
4: Pearson et al. (1981), 5: Unwin et al. (1985), 6: Abraham et al. (1996)

© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999
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