Measurement of the parallax of PSR B0950+08 using the VLBA

A new technique has been developed to remove the ionosphere's distorting effects from low-frequency VLBI data. By fitting dispersive and nondispersive components to the phases of multifrequency data, the ionosphere can be effectively removed from the data without the use of a priori calibration information. This technique, along with the new gating capability of the VLBA correlator, 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 +/- 0.4 mas yr(-1), mu(delta) = 29.5 +/- 0.5 mas yr(-1)) and parallax (pi = 3.6 +/- 0.3 mas). This puts the pulsar at a distance of 280 +/- 25 pc, about twice as far as previous estimates, but in good agreement with models of the electron density in the Local Bubble.