6.2. The VLBI Jet

The core of 3C 273 (3C 273 B as it was called in the 1960s) is a very bright radio source. The presence of the jet described above at large angular distances showed that the source is not point like, but has an interesting geometry. Both facts made 3C 273 a prime source for high resolution radio observations as these became possible by using together telescopes spanning approximately the size of the Earth (Very Long Baseline Interferometry, VLBI in short). Early observations used few telescopes and did not produce maps but were able to measure whether or not the source is extended at certain angular scales. [Broten et al. 1967] and [Clark et al. 1967] showed thus that 3C 273B has an angular size less than 0.005" at a wavelength of 18cm. Structure on the scale of milli arcseconds (mas in the following) has been found by [Knight et al. 1971] and [Cohen et al. 1971] who also note a difference between their results that can be interpreted by a change in angular size of the source. Radio data (visibility functions rather than maps) confirmed the reality of the changes and revealed a steady expansion of the source between 1970 and 1977 [Cohen et al. 1977]. Study of the location of the minima in the visibility curves showed an apparent expansion velocity of 5.2c (H₀ = 55km/s/Mpc) [Cohen et al. 1979]. Maps were also obtained then, showing for the first time a real jet structure at a position angle of -117°, not aligned with the larger scale jet described above [Readhead et al. 1979]. Subsequent maps at higher angular resolutions and using more antennae than previously, thus improving the image quality, showed that the local maximum of the jet moves away from the core. The distance from the core to the main jet feature had increased from 6mas in 1977.5 to 8mas in 1980.5 [Pearson et al. 1981] corresponding to an apparent expansion velocity of 9.6 ± 0.5c (H₀ = 55km/s/Mpc).

More recent VLBI observations have continued the work done at cm wavelengths, have used higher frequency observations to increase the angular resolution and have improved on the dynamical range to study weaker features. These modern data have confirmed the picture described above and added several new features.

A set of several VLBI observations in the 1980s has revealed that new jet components (often called blobs) appear every few years. These components can be followed from one observation to the next and their projected trajectories mapped. It is thus possible to trace back each component to the time of zero separation from the core [Krichbaum et al. 1990]. One of the component was observed to be thus "born" shortly after a violent synchrotron outburst that had been observed at wavelengths as short as the visible band in March 1988 (see above; [Courvoisier et al. 1988]). This close association suggests that in general new components in the jet follow synchrotron outbursts. This is indeed claimed in a study of [Abraham et al. 1996] in which the ejection time of 8 components is computed and qualitatively compared to single dish light curves. We show in Fig. 11 the high frequency radio light curves and a near infrared light curve available to us (see above) and the epochs of appearance of new jet components as computed by [Abraham et al. 1996]. Whereas it seems clear that the ejection of C9 is associated with the infrared outburst discussed above, no clear statement can be made for the preceding ejections.

Figure 11. millimetre and infrared light curves and dates of appearance of new VLBI jet components (see the text). The components are labeled as in . The epochs of ejection of the components are from . The uncertainty in the ejection epochs are shown by a short range.

[Abraham et al. 1996] have also correlated the epoch of ejection of components with the radio light curve at 22GHz they claim that the ejection times of all components are related to increases in the radio flux. They do not, however, provide a quantitative assessment of this relationship. Flux increases are indeed expected to be associated with the appearance of new jet components if these are new ejecta that become optically thin as they move away from the core. A further possible link has been established by [Krichbaum et al. 1996] between the ejection of the knots and the high energy activity of 3C 273 as evidenced by ECRET data.

The VLBI observations quoted in [Krichbaum et al. 1990] were made at 43GHz. VLBI observations at even higher frequencies (100GHz) were obtained by [Bååth et al. 1991]. These data reach a resolution of 50micro seconds of arc, illustrating the power of the technique. Using H₀ = 50km/s/Mpc this angular resolution corresponds to a linear scale of 510¹⁷cm at the distance of 3C 273. This is to be compared with the gravitational radius of a 10¹⁰ solar masses black hole, which is 310¹⁵cm. In other words, modern VLBI observations are capable of resolving structures in the radio data of 3C 273 down to 100 gravitational radii. This effort to obtain maps at higher frequencies is being pursued (see e.g. [Krichbaum et al. 1997]).

High angular resolution VLBI data reveal that the angle at which the jet emerges from the core is significantly different at the hundred micro arcsecond scale (-119°) from that observed at the mili arcsecond scale (-130°) or at longer scales (-137°) ([Bååth et al. 1991] and references therein). [Bååth et al. 1991] interpret this result as being due to either deflection of the jet or (but this is in a sense equivalent) to changes in the speed of the jet. This must be put in parallel with the observation of [Krichbaum et al. 1990] who report that the velocities of the individual knots are different (from 4 ± 0.3 to 8 ± 0.2 times the velocity of light).

Another type of improvement in the knowledge of the jet was brought about by investigations with a higher dynamical range. Such observations are reported in [Davis Unwin & Muxlow1991]. Two important results follow from their data. The superluminal motions observed at small distances from the core extend to at least 240pc (H₀ = 50km/s/Mpc). The velocity at large distances is only marginally less than closer to the core. The second result is that no counter jet is detected. The brightness ratio between a jet and an intrinsically identical counterjet is given by

Using a spectral index of 0.8 [Davis Unwin & Muxlow1991] deduce from the observed lower limit on this ratio that 0.95. This velocity is close to that obtained from the superluminal expansion (see below). The data available is therefore still compatible with the presence of a counter jet of similar properties to the one observed but unobserved due to its relativistic motion away from us. A further improvement of the dynamical range by a factor of a few would provide an important set of data on the intrinsic properties of an eventual counter jet.

The intrinsic velocity of a relativistic jet can be deduced from the apparent proper motion in the following way (the original model is due to [Blandford McKee & Rees 1977]):

Figure 12. The VLBI Jet of 3C 273 at two different epochs in 1994 and 1995 observed at 86GHz. (courtesy T. Krichbaum.)

Assume that a photon is emitted by a blob of the jet that has traveled during t at the velocity v. The difference in arrival time of this photon and one that originated from the base of the jet at the time of departure of the blob t is

The motion of the blob a perpendicular to the line of sight seen by an observer far away is

It is easily seen from both expressions that superluminal motion can be observed for v close to c and cos close to one and that the angle for which the transverse velocity is maximum for a given intrinsic velocity is given by