The mass of a millisecond pulsar

We report on nearly 2 years of timing observations of the low-mass binary millisecond pulsar PSR J1909 - 3744 with the Caltech-Parkes-Swinburne Recorder II, a new instrument that gives unprecedented timing precision. Daily observations give a weighted rms residual of 74 ns, indicating an extremely low level of systematic error. We have greatly improved on the previous parallax and proper motion measurements of PSR J1909 - 3744, yielding a distance of 1.14(-0.03)(+0.04) kpc and transverse velocity of 200(-6)(+7) km s(-1). The system's orbital eccentricity is just (1.35 +/- 12) x 10(-7), the smallest yet recorded. Since their discovery, the masses of the rapidly rotating millisecond pulsars have remained a mystery, with the recycling hypothesis arguing for heavy objects, and the accretion-induced collapse of a white dwarf more consistent with neutron stars less than the Chandrashkar limit. Fortuitously, PSR J1909 - 3744 is an edge- on system, and our data have allowed the measurement of the range and shape of the Shapiro delay to high accuracy, giving the first precise determination of a millisecond pulsar mass to date, m(p) =1.438 +/- 0.024 M-circle dot. The mass of PSR J1909 - 3744 is at the upper edge of the range observed that the production of millisecond pulsars is possible with the accretion of < 0.2 M-circle dot.