Towards Determination of Galactic Rotation Curve through Phase- Referencing VLBI Observations of Astronomical Maser Sources

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VERA (VLBI Exploration of Radio Astronomy) is a new VLBI array of National Astronomical Observatory mainly aimed at study of the structure and dynamics of our Galaxy through high-precision phase-referencing VLBI astrometry of astronomical maser sources. The project was recently funded and the four station array is under construction. In the present study, we conducted 22 GHz switching phase-referencing VLBI observations of two pairs of Galactic water maser sources and adjacent extragalactic continuum radio sources using VLBA of NRAO as the first step towards determination of the rotation curve of the Galaxy outside the Solar Circle. The primary purpose of the study was to confirm basic concepts of VERA project on the basis of the real VLBI data and to prepare to VERA observations. The two pairs of radio sources were observed twice with a month interval in 1998 and 1999, respectively, in the fast-switching phase-referencing mode with 40 second switching cycle. Analyses of the observed results which were mainly performed by Kazuya Hachisuka, a graduate student of the head investigator, revealed(1) the large fluctuation in the VLBI fringe phase due mostly to the water vapor in the turbulent troposphere was successfully compensated in the phase-referencing, and the residual fluctuation in 40-second-integrated phase difference observable was as small as 10 degrees (which corresponds to 50 microarcsecond angular precision with 2000 km projected baseline) ,(2) celestial displacements of the two maser sources with assumed distances of roughly 7 kpc and 2 kpc, respectively, during a month were clearly detected for the first time, and the were mostly consistent with the expected proper motions due to the combained effects of the Galactic rotation, annual parallax and solar motion.The results confirmed the eligibility of the phase-referencing VLBI observations with VERA in the high-precision maser astrometry with 10 microarcsecond-level accuracy.