VLBA Observations of Astrometric Reference Sources in the Galactic Center

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Abstract
We report here on multifrequency VLBA observations of three extragalactic sources within 1° of the Galactic center. These sources have been used as astrometric reference sources for VLA and VLBA determinations of the proper motion of Sagittarius A*, the compact nonthermal radio source in the Galactic center. Each source has a main component with a brightness temperature in excess of 107.5 K, confirming that the sources are active galactic nuclei. The sources have simple structure that can be characterized by one or two Gaussian components. The frequency dependence of the structure indicates that the positions of Sgr A* determined by the VLA astrometry of Backer & Sramek at 4.8 and 8.4 GHz should have an offset of ~2 mas. This offset is in the same direction as the 5 mas shift measured by Backer & Sramek. The structure is unlikely to bias the published 43 GHz VLBA results of Reid et al. Motions of components in the calibrator sources could lead to errors in the proper motion of Sgr A* on the order of a few km s-1. All three sources show frequency-dependent structure consistent with scattering significantly stronger than that of the Galactic scattering model of Taylor & Cordes but significantly weaker than that of the hyperstrong Galactic center scattering. Combined with other observations, this suggests the existence of a new component of Galactic scattering located several kpc from the Galactic center.
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