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Interleaved VLBI phase-delay observations of two sources which lie near each other on the plane of the sky can determine their relative positions with sub-milliarcsecond uncertainties. A series of such observations well spread throughout the seasons of a pulsar and one or more extragalactic reference sources can yield the position, proper motion, and parallax of the pulsar, the latter providing a model-independent distance. These results could be used to check the calibration of the dispersion-based galactic distance scale, to investigate the properties of the free-electron density distribution in the solar neighborhood, and to study the spatial distribution and kinematics of the pulsars. We present an overview of our ongoing VLBI pulsar parallax program and discuss our results for the two pulsars (PSR 2021+51 and PSR 1929+10) which we have observed in four epochs since 1991 July.
We acknowledge support from NASA Grant NGT-50663 (RMC) and NSF Grants AST89-02087 (NB, IIS) and AST93-03527 (RMC, IIS); research at York University is partially supported by NSERC.
