ON THE ORBITAL PERIOD CHANGE OF THE BINARY PULSAR PSR-1913+16

We reexamine the theoretical significance of the raw observational parameter called the "rate of orbital period change," P(b)obs, of the binary pulsar PSR 1913 + 16. We show that the current precision (approximately 0.8%) on the determination of P(b)obs makes it necessary to take explicitly into account the effects of the galactic accelerations of the pulsar and the Sun, and that of the proper motion of the pulsar. Several other possible contributions to P(b)obs are (re)examined and found negligible. As the value of the galactic contribution to P(b)/P(b) depends explicitly on the distance to the pulsar, say d, we have been led to reexamine the determination of d from dispersion measurements. We find that recent progress in H I absorption measurements in the first galactic longitude quadrant allows one to constrain the mean electron density along the line of sight to PSR 1913 + 16 (l = 50-degrees) to the range nBAR(e)(50-degrees) = (]6.39 +/- 0.93][R(o)/1 kpc])-1 cm-3, where R(o) is the galactocentric distance of the Sun. This value for nBAR(e) is smaller than the "standard" one and leads to a galactic-reduced distance to PSR 1913 +/- 16, d/R(o) = 1.08 +/- 0.16. After subtraction of the galactic effects, the latest experimental results yield a 0.8% confirmation of the general relativistic prediction: P(b)obs-gal/P(b)GR = 1.0081 +/- 0.0022(galactic) +/- 0.0076(observational). Alternatively, this result yields an upper bound to the rate of change of Newton's gravitational constant, G/G = (1.10 +/- 1.07) x 10(-11) yr-1, which, in the long term, may be limited to the +/- 3 x 10(-12) yr-1 level of precision because of the uncertainties in the values of the galactic constants, R(o) and v(o).