Fundamental astrometry and millisecond pulsars

Using a new data acquisition system designed specifically for timing millisecond pulsars, the authors have obtained 2 yr of data on PSRs 1937+21 and 1953+29 and 1 yr on PSR 1855+09. For PSR 1937+21, pulse arrival times have been combined with previously published data for two additional years in an attempt to detect annual parallax and proper motion. The authors obtain a limit π < 0.4 mas for the parallax and, in the nearly inertial reference frame of the PEP740R solar system ephemeris, a surprisingly small proper motion in galactic longitude, μl = -0.6±0.3 mas yr-1. Interstellar scintillation parameters derived from the same set of observations are consistent with a pulsar moving with transvese velocity less than 40 km s-1 relative to interstellar gas along the line of sight. For the other two millisecond pulsars, PSRs 1855+09 and 1953+29, the new timing data yield improved estimates of celestial coordinates and rotation and orbital parameters.