Measurement of the Parallax of PSR B0950+08 Using the VLBA

A new technique has been developed to remove the ionosphere's distortingeffects from low frequency VLBI data. By fitting dispersive and non-dispersivecomponents to the phases of multi-frequency data, the ionosphere can beeffectively removed from the data without the use of {\em a priori} calibrationinformation. This technique, along with the new gating capability of the VLBAcorrelator, was used to perform accurate astrometry on pulsar B0950+08,resulting in a much improved measurement of this pulsar's proper motion($\mu_{\alpha} = -1.6 \pm 0.4$ mas/yr, $\mu_{\delta} = 29.5 \pm 0.5$ mas/yr)and parallax ($\pi = 3.6 \pm 0.3$ mas). This puts the pulsar at a distance of$280 \pm 25$ parsecs, about twice as far as previous estimates, but in goodagreement with models of the electron density in the local bubble.Comment: 5 pages, Latex with AASTEX. Accepted for publication in Ap