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                              G C N E W S
                             * Newsflash *

            - The Newsletter for Galactic Center Research -

gcnews@mpifr-bonn.mpg.de             http://www.mpifr-bonn.mpg.de/gcnews
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Vol. 13, No. 10                                              May 3, 2001


Recently submitted papers:
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1) VLBA Observations of Astrometric Reference Sources in the Galactic 
   Center (Bower et al., ApJ)
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Email    : gbower@palan.berkeley.edu
Title    : VLBA Observations of Astrometric Reference Sources in the 
           Galactic Center
Author(s): Geoffrey C. Bower(1), Donald C. Backer(2), \& Richard A. 
           Sramek(1)
Institute: (1) National Radio Astronomy Observatory, P.O. Box O, 1003 
           Lopezville, Socorro, NM 87801; 
           gbower@nrao.edu,rsramek@nrao.edu (2) Astronomy Department \& 
           Radio Astronomy Laboratory, University of California, 
           Berkeley, CA 94720; dbacker@astro.berkeley.edu
Paper    : ApJ, Sept 2001, in press
EPrint   : astro-ph/0105016

Abstract:
We report here on multi-frequency VLBA observations of three 
extragalactic sources within 1 degree 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 10^7.5 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 (1999) 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 (1999). The structure is unlikely to bias the 43 GHz VLBA 
results of Reid et al. 1999. 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 which is significantly stronger 
than that of the Galactic scattering model of Taylor & Cordes (1993) 
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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    Angela Cotera                       Heino Falcke & Sera Markoff
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