HIGH-PRECISION TIMING OBSERVATIONS OF THE MILLISECOND PULSAR PSR-1937+21 AT NANCAY

Observations of the millisecond pulsar PSR 1937+21 at Nancay Radio Obs ervatory provide the densest series of times of arrival (TOA) and of f lux densities on this pulsar. They were measured on 525 individual dat es over 4.5 years from 1988 December, 22. Comparison between the astro metric parameters fitted to the Nancay and Arecibo independent timing series shows that they are consistent at the level of 4 times the form al uncertainty (sigma) for all parameters except the period (7 sigma) but that is likely caused by the different TAI time scales used in the 2 data sets. The post-fit TOA residuals of the two series are charact erized by the same rms, 0.36 mu s at 1.4 GHz, if the times of arrival with the highest flux densities (> 345 mJy) are selected at Nancay. We find also that there is a high correlation between the TOA residuals and the flux density of this pulsar as expected if refractive scintill ation is important in the ionized interstellar medium. Finally, when t he second period derivative is not solved for in the Nancay data, the post-fit TOA residuals are dominated by a low-frequency noise as alrea dy discovered in the Arecibo data on PSR 1937+21. In this context, we study the effects of the large minor planets recently discovered at th e edge of the Solar System (Chiron, Phollus, 1992 QB1) and not include d in the ephemerides used for the analysis. We conclude that they are negligible. However, the hypothetical more massive tenth Planet (Plane t X) might already be partially responsible for the low-frequency nois e in PSR 1937+21 residuals. We show that, if this planet exists, its s ignature will be large over 20 years of high-precision timing of the m illisecond pulsars PSR 1937+21, PSR 1855+09 and PSR 1821-24