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 non-dispersive components to the phases of multi-frequency data, the ionosphere can be effectively removed from the data without the use of {\em 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 (${\mu }_{\alpha }=-1.6\pm 0.4$ 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 good agreement with models of the electron density in the local bubble.