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Astron. Astrophys. 336, 385-392 (1998)

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1. Introduction

Astrometric VLBI observations of extragalactic radio sources are the most effective tool to define a quasi-inertial reference frame (e.g., Johnston et al. 1995). With phase-delay astrometry, bounds on the proper motion of several radio sources have been measured (e.g., Bartel et al. 1986; Marcaide et al. 1994). In these studies, the variability of the structure of the radio sources, and the consequent difficulty in locating a fixed reference point on the radio map, dominate the astrometric error budget (up to 80% of the overall standard error of the proper-motion measurement quoted by Marcaide et al. 1994).

When proper motion is measured, there is no guarantee that the motion corresponds to that of the center of mass of the target source, since the structure of the reference source and its variability introduce uncertainty in the registration of maps from multiple epochs of observation. Registration is more reliable when the reference source has more compact structure or when the reference and target sources possess jets oriented more nearly orthogonal to each other. The latter is the case for the radio source pair 1928+738 and 2007+777. The inner parts of their superluminal jets are oriented with position angle (PA) [FORMULA] and [FORMULA], respectively, and the relative motions detected in right ascension and declination can be assigned to 1928+738 with reasonable confidence.

In this paper we report a comparison of two determinations of the relative position of the radio sources 1928+738 and 2007+777, one each made at 5 GHz and at 8.4/2.3 GHz (simultaneously). The 5 GHz observations were reported by Guirado et al. 1995a (hereafter G95); therefore we focus here on the 8.4/2.3 GHz observations. We briefly describe the astrometric analysis and compare the resultant relative position with that obtained at 5 GHz.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998