VLBA Observations of Astrometric Reference Sources in the Galactic Center

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 \sim 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.

12 pages, 4 figures, accepted for publication in ApJ

References in corpus (5)

• The Accelerations of Stars Orbiting the Milky Way's Central Black Hole
• The Proper Motion of SgrA*: I. First VLBA Results
• Proper Motion of the Compact, Nonthermal Radio Source in the Galactic Center, Sagittarius a*
• The Linear Polarization of Sagittarius A* I. VLA Spectro-polarimetry at 4.8 and 8.4 GHz
• The Sizes of 1720 MHz OH Masers: VLBA and MERLIN Observations of the Supernova Remnants W44 and W28

Cited by in corpus (16)

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• Detection of the Intrinsic Size of Sagittarius A* through Closure Amplitude Imaging (includes Online Supporting Material)
• The Intrinsic Size of Sagittarius A* from 0.35 cm to 6 cm
• Disc-jet coupling in the 2009 outburst of the black hole candidate H1743-322
• Interstellar magnetic fields in the Galactic center region
• Magnetic field near the central region of the Galaxy: Rotation measure of extragalactic sources
• Localizing Sagittarius A* and M87 on Microarcsecond Scales with Millimeter VLBI
• Extragalactic sources towards the central region of the Galaxy
• Locating the intense interstellar scattering towards the inner Galaxy
• Radio polarimetry of Galactic centre pulsars
• The Formaldehyde Masers in Sgr B2: Very Long Baseline Array and Very Large Array Observations
• Density of warm ionized gas near the Galactic Center: Low radio frequency observations
• An Infrared Imaging Survey of the Faint Chandra Sources near the Galactic Centre
• A Measurement of Proper Motions of SiO Maser Sources in the Galactic Center with the VLBA