We have used 5 GHz and 8.4/2.3 GHz VLBI observations of the radio sources 1928+738 and 2007+777 to deduce a change in the position of the peak of brightness of 1928+738 of 0.740.22 mas along PA between 1985.77 and 1988.83. This shift is attributed mostly to the presence of a new component emerging near epoch 1988.83, and, to an unknown, but lesser, degree, to frequency-dependent opacity effects. The measured change in separation could be part of a recurrent pattern of motion of the peak of brightness produced by components intermittently ejected from the core.
Our results indicate that at centimeter wavelengths the peak of brightness of 1928+738 does not correspond to its core, but to moving components. Therefore, even by comparing maps at similar frequency bands, it would be difficult to set a useful upper bound to the stability of the core, as the uncertainties would be dominated by those in the identification of the core in the maps. As for other sources, e.g., 4C 39.25 (Guirado et al. 1995b) and 1038+528 (Marcaide et al. 1994; Rioja et al. 1997), the limiting factor in the determination of relative position and proper motion is the uncertainty in the consistent identification of a reference point in the radio source maps from different epochs. Observations at higher resolution (i.e., millimeter VLBI and/or space VLBI) should alleviate this problem, since they offer the possibility of matching the resolution of the radio source maps with the precision of the astrometry; in such observations, the selection of the reference point should be more straightforward and less uncertain. In addition, observations at two or more higher frequencies should allow the detection of inverted spectral features, associated with the core.
© European Southern Observatory (ESO) 1998
Online publication: July 7, 1998