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• High precision difference astrometry applied to the triplet of S5 radio sources B1803+784/Q1928+738/B2007+777 ,
  E. Ros, J. M. Marcaide, J. C. Guirado, M. I. Ratner, I. I. Shapiro, T. P. Krichbaum, A. Witzel, & R. A. Preston
  Astronomy & Astrophysics, 348, 381-393 (1999)
  
  • Abstract We determined the separations of the radio sources in the triangle formed by the BL Lac objects 1803+784 and 2007+777, and the QSO 1928+738 from intercontinental interferometric observations carried out in November 1991 at the frequencies of 2.3 and 8.4 GHz. Difference phase-delay astrometry yielded the following separations:
    Delta alpha_{(1803+784)-(1928+738)}=-1^h 27^m 2^s.811256+/-0^s000062
    Delta delta_{(1803+784)-(1928+738)}= 4^o 30' 2''.44833+/-0''.00012
    Delta alpha_{(1928+738)-(2007+777)}=-0^h 37^m 42^s.503305+/-0^s.000033
    Delta delta_{(1928+738)-(2007+777)}=-3^o 54' 41''.67756+/-0''.00013
    Delta alpha_{(2007+777)-(1803+784)}= 2^h 4^m 45.^s.314561+/- 0^s.000052
    Delta delta_{(2007+777)-(1803+784)}=-0^o 35' 20''.77077+/-0''.00013
    We successfully connected differenced phase delays over 7^o on the sky at 8.4 GHz at an epoch of maximum solar activity. The effects of the ionosphere on these VLBI data were mostly removed by estimates of the total electron content from observations of GPS satellites. The comparison of the estimated separation of QSO 1928+738 and BL 2007+777 with previous such estimates obtained from data at different epochs leads us to a particular alignment of the maps of QSO 1928+738 at different epochs relative to those of BL 2007+777, although with significant uncertainty. Our result shows that the jet components of QSO 1928+738 have a mean proper motion of 0.32+/-0.10 mas/yr and also suggest an identification for the position of the core of this radio source.

Eduardo Ros ros@mpifr-bonn.mpg.de

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