## 3. Local astrometryThe VLBI results are expressed in the equatorial system of coordinates (right ascensions and declinations), which is linked to the Earth's rotation axis and has no particular relationship with the source geometry. On the other hand, most of the known activity of quasars takes the form of jets, i.e., aligned moving emissive structures that induce motions of the observed emission centre relative to a fixed background. We therefore considered for each source a local system of axes centered at the mean equatorial coordinates of the source for the whole observation span (1988-1999): the x-axis is in the direction of maximum standard deviation of the 0.5 year weighted averaged coordinates over 1988-1999, and the y-axis forms a direct rectangular system with it. Fig. 1 shows the ellipses of standard deviations of the 0.5 year averages, in the equatorial reference frame.
Note that there is no particular sign of a preferred N-S direction for the maximum standard deviation, suggesting that the contribution of the ill-modelled propagation delay due to the wet troposphere is not dominant in our sample of selected sources. Table 3 gives, for the 16 sources under study, the directions of maximum variability (0 and 180 degrees correspond to the direction of right ascensions, 90 degrees to the direction of declinations), and the standard deviations along the two source-fixed axes. For reference, Table 3 also gives the structure indices in X and S computed by Fey & Charlot (1997, 2000). These indices qualify the nuisance of the structure in astrometric positioning of the source. They range from 1 for very little disturbance through 4 for disrupting effect.
Fig. 2 shows the 2D source displacements (yearly values) of the same sources over 1991-1999 in the source-fixed frames. The error bars represent the standard error of the yearly averaged coordinates.
© European Southern Observatory (ESO) 2000 Online publication: July 13, 2000 |