HIGH-PRECISION TIMING OF MILLISECOND PULSARS .1. ASTROMETRY AND MASSES OF THE PSR-1885+09 SYSTEM

Timing observations of several millisecond pulsars are being made on a regular basis, generally twice a month, at the Arecibo Observatory. We describe the observing method and procedures of data analysis used for one particular object, PSR 1855 + 09, a 5.4 ms pulsar in a 12.3 day orbit. Measurements have now been obtained on a total of 94 days over a 4.4 yr span, with timing accuracies ranging from about 2-mu-s to 0.4-mu-s. We use a general relativistic timing model to obtain improved values for all previously determined parameters of the pulsar and its orbit. In addition, we find the annual parallax of the system to be pi = 1.2 +/- 0.5 mas and the proper motion to be mu = 6.07 +/- 0.20 mas yr-1, directed nearly along the galactic plane. For the first time, we present measurements of two orbital parameters quantifying the magnitude and shape of the general relativistic "Shapiro delay" in the orbiting system. The parameter values show the orbital inclination to be nearly 90-degrees, and they provide mass estimates for the pulsar and companion star, respectively m1 = 1.27(-0.15)+0.23 M. and m2 = 0.233(-0.017)+0.026 M.. Both masses are in good agreement with theoretical predictions based on neutron star physics and evolutionary models of the PSR 1855 + 09 system.