4. Result and discussion
The relative positions of the radio sources 1928+738 and 2007+777 at the two epochs are represented in Fig. 4 with their standard deviations. This figure shows a significant net change with time of the relative declination of the selected fiducial points of the sources, while the right ascension change is not significantly different from zero. There are several phenomena that could, individually or in combination, explain such a change in separation: 1) opacity effects on the apparent location of the fiducial point, 2) a shift with respect to the center of mass of a selected fiducial point, due to the blending of the core with an emerging component, 3) misidentification of the selected reference feature at different epochs, and 4) motion of the center of activity (in particular, binary black-hole models of 1928+738 predict a precession of the core with amplitude 0.1 mas and period of 3 yr (Hummel et al. 1992); however, such an amplitude would be difficult to detect reliably with our data due to the possible presence of optical-depth effects which could not be separably detected even when comparing positions obtained at our two different frequency bands). We discuss these possibilities below.
What is the correct "registration" of the maps from each of the two sources for the two epochs of observation? In particular, to which source or sources do we attribute the change in separation that we obtained from the astrometry? It seems likely that the change in declination is due to changes in the inner structure of 1928+738, since this source appears to have a southward-directed jet with moving components, and transverse motions in the eastward-directed jet of 2007+777 are less likely. We interpret the null result for the change in relative right ascension as the absence of apparent motion in the region of 2007+777 near its core (as opposed, e.g., to both sources exhibiting similar motions in the same direction). Hence, we assume that the entire relative shift is due to changes within 1928+738, while 2007+777 behaves as a stationary reference source to within the limits set by our standard errors. The adopted registration is shown in Fig. 5 and Fig. 6 for 1928+738 and 2007+777, respectively. The corresponding change in the relative position of the peaks of brightness in the three years between 1985.77 and 1988.83 can be expressed as the vector change in position of the reference point in 1928+738 with respect to that in 2007+777, i.e., a vector of magnitude 0.740.22 mas along PA .
Such an apparent motion of the reference point in 1928+738 can be explained in terms of changes in the source structure between the two epochs and/or of changes in opacity effects between the two frequency bands. It seems unlikely that this "motion" corresponds to that of a particular component; it would need to be moving towards the core, which would be difficult to explain within standard relativistic jet models. More likely is that the observed motion is a consequence of our failing to identify the same physical feature in 1928+738 as the reference point at the two epochs, i.e., that the peak of brightness corresponds to different jet components in each map. Supporting evidence for this hypothesis is the presence near 1988.83 of an emerging component in the maps of 1928+738 at 22 GHz (Hummel et al. 1992). This component could be associated with component X2, partially blended with the core in our 8.4 GHz map. The peak of the 5 GHz image (C2) in this scenario corresponds to a different jet component located further down the jet than component X2; it is unfortunately unclear whether the feature C1, elongated toward the north, is the core. According to this hypothesis, the shift detected in our astrometry should be part of a continuous motion of the peak of brightness. In this scenario, the peak of brightness moves as the brightest component travels down the jet (southwards for 1928+738); this motion would continue until the component fades and/or a new, and hence brighter, component is ejected; the peak of brightness would then be a blend of the new emerging component and the core, and a rapid shift of this peak towards the core (northwards for 1928+738) would occur. Such a rapid shift likely caused most or all of the difference in position we observed. For our maps, which were obtained at different radio frequencies for the two epochs, we would expect some contribution to the observed change in separation of the peak of brightness due to opacity effects that depend on frequency. However, given the relatively small difference between the frequencies of the maps (5 and 8.4 GHz), the observed change in separation between the sources (0.7 mas) is probably too big to be produced solely by opacity effects (Marcaide & Shapiro 1984; Lara et al. 1996). Thus, although the observed change in separation is likely a combination of temporal and spectral changes of the source structure, the contribution of the former is probably considerably greater than that of the latter.
We can predict the position of component C2 in our 8.4 GHz map using the proper motion for the 5 GHz components of Schalinski (1990): 0.560.04 mas/yr; however, at the predicted position, the 8.4 GHz map shows just a trace of emission, labelled as X4 in Figs. 1 and 5, only 1% of the peak. This emission could correspond to C2, if C2 had faded rapidly as it traveled southwards.
© European Southern Observatory (ESO) 1998
Online publication: July 7, 1998