| Using Very Long Baseline Interferometry, I have measured the parallax of two pulsars and have observed absence of relative motion between members of pairs of quasars. Observations were made over a ten-month period at 18 cm, with antennas and data recorders at Green Bank, WV, Owens Valley, CA, and Arecibo, PR. Each program source was observed alternately with a nearby reference source. The recorded signals were processed to produce interference fringes, which were corrected for the effects of polar motion, the troposphere and the ionosphere. The fringe phases of each program source were then differenced with those of its reference, reducing residual systematic errors. Variations of this difference between epochs reflected program source proper motion and parallax.; I obtain a parallax of 7.8 (+OR-) 0.6 mas for PSR 0950 + 08, corresponding to a distance of 130 (+OR-) 10 pc, and a parallax of 1.8 (+OR-) 0.4 mas for PSR 0823 + 26, corresponding to a distance of 560 (+OR-) 130 pc. In combination with published measurements of pulse dispersion for these pulsars, these yield average free electron densities of 0.023 (+OR-) 0.002 cm('-3) along the line of sight to PSR 0950 + 08 and 0.035 (+OR-) 0.008 cm('-3) along the line of sight to PSR 0823 + 26. These are in good agreement with free electron densities measured by other techniques along lines of sight 1 kpc or more in length. Both parallax measurements are subject to systematic errors and questions of interpretation which have not yet been fully explored. |
