HIGH-PRECISION TIMING OBSERVATIONS OF THE MILLISECOND PULSAR PSR1821-24 AT NANCAY

High-precision timing observations of the millisecond pulsar PSR 1821- 24 have been conducted on 305 individual dates at the Nancay radiotele scope at 1.4 GHz between March 2 1989 and July 21 1993. The Time Of Ar rival residuals after the standard fit of the pulsar parameters are ch aracterised by a rms of 2.78 mu s. This dense and precise timing serie s has allowed the first determination of the apparent second period de rivative P for PSR 1821-24 and of its proper motion. We find that the observed quasi-cubic variation (i.e. the apparent P) of the post-fit r esiduals matches the level of low-frequency noise predicted by the emp irical relationship of Arzoumanian et al. (1994) for the rotation irre gularities of classical pulsars. We conclude that the millisecond puls ar PSR 1821-24, similarly to PSR 1937+21, exhibits instrinsic rotation irregularities, unless P is a jerk (it) induced during a close encoun ter in the globular cluster M28. This pulsar is the youngest known mil lisecond pulsar according to a characteristic age (1/2 P-0/P-0) of 30 x 10(6) yr. Relatively young millisecond pulsars, like PSR 1821-24 and PSR 1937+21, might be prone to rotation irregularities while old mill isecond pulsars like PSR 1855+09 an more stable. We find a discrepancy between the optical proper motion of M28 and our timing proper motion of PSR 1821-24 larger than the pulsar escape velocity in the cluster. Reconciling these two proper motions is important for the kinematics of M28 and the study of the gravitational potential of the Galactic di sk and bulge. Finally, we present the daily timing observations of PSR 1821-24 conducted every year since 1989 when the solar corona interve nes between the pulsar and the Earth at Christmas time. We have used t hese timing observations along a single cut through the corona to fit a spherically symmetric model of its electron density n(e) = n(0)(r/r( 0))(-alpha). With a slightly more sophisticated model for the coronal electron density, precise timing observations of PSR 1821-24 during th ese periods over a complete solar cycle could monitor the global flatt ening expected for the coronal plasma at the solar minimum.