TIMING MILLISECOND PULSARS (ASTROMETRY)

UHF timing observations of the "millisecond pulsar" PSR 1937+21 have been conducted with the 73000 m('2) telescope of the Arecibo Observatory since October 1984 using a multichannel signal averager built for the purpose and described herein. A two-hour dual-frequency observation measures the pulsar rotation phase at one instant with 275 ns accuracy. The timing results determine the pulsar's celestial proper motion and place a 2(sigma) lower limit of 2 kpc on its distance. The integrated electron density along the line of sight to the pulsar is shown to vary by 1 part in 10('5) on time scales of order 6 months. Time residuals from the astrometric fit have the properties of a flicker phase modulation, a type of noise indicative of measurement errors (probably instrumental) rather than pulsar rotation instabilities or gravitational waves. The local energy density in gravitational waves of period (TURN)3 years must be less than 3 x 10('-35) g cm('-3).; The other two known millisecond pulsars, PSRs 1953+29 and 1855+09, have also been timed. The secular increase in the rotation period of PSR 1953+29, the smallest such increase yet measured, implies an unusually small surface magnetic field of 4 x 10('8) G for this pulsar, very close to that derived for PSR 1937+21, despite the fact that PSR 1953+29 is in a binary system and PSR 1937+21 is not. These pulsars probably have similar histories involving spin-up by accretion. The rate of period increase of PSR 1855+09 may be even smaller, but a few more months of data are needed to determine it precisely. PSR 1855+09 is potentially nearly as good a clock as PSR 1937+21.