MICROARCSECOND VLBI PULSAR ASTROMETRY WITH PSRπ. I. TWO BINARY MILLISECOND PULSARS WITH WHITE DWARF COMPANIONS

Model-independent distance constraints to binary millisecond pulsars (MSPs) are of great value to both the timing observations of the radio pulsars and multiwavelength observations of their companion stars. Astrometry using very long baseline interferometry (VLBI) can be employed to provide these model-independent distances with very high precision via the detection of annual geometric parallax. Using the Very Long Baseline Array, we have observed two binary MSPs, PSR J1022+1001 and J2145-0750, over a two-year period and measured their distances to be 700(10)(+14) pc and 613(-14)(+16) pc respectively. We use the well-calibrated distance in conjunction with revised analysis of optical photometry to tightly constrain the nature of their massive (M similar to 0.85 M-circle dot) white dwarf companions. Finally, we show that several measurements of the parallax and proper motion of PSR J1022 + 1001 and PSR J2145-0750 obtained by pulsar timing array projects are incorrect, differing from the more precise VLBI values by up to 5 sigma. We investigate possible causes for the discrepancy, and find that imperfect modeling of the solar wind is a likely candidate for the errors in the timing model given the low ecliptic latitude of these two pulsars.