SIMBAD references

High-precision timing observations of the millisecond pulsar PSR 1821-24 have been conducted on 305 individual dates at the Nancay radiotelescope at 1.4GHz between March 2 1989 and July 21 1993. The Time Of Arrival residuals after the standard fit of the pulsar parameters are characterised by a rms of 2.78µs. This dense and precise timing series has allowed the first determination of the apparent second period derivative d²P/dt² for PSR 1821-24 and of its proper motion. We find that the observed quasi-cubic variation (i.e. the apparent d²P/dt²) of the post-fit residuals matches the level of low-frequency noise predicted by the empirical relationship of Arzoumanian et al. (1994) for the rotation irregularities of classical pulsars. We conclude that the millisecond pulsar PSR 1821-24, similarly to PSR 1937+21, exhibits instrinsic rotation irregularities, unless d²P/dt² is a jerk ({dot}(a)) induced during a close encounter in the globular cluster M28. This pulsar is the youngest known millisecond pulsar according to a characteristic age (1/2P₀/{dot}(P)₀) of 30x10⁶yr. Relatively young millisecond pulsars, like PSR 1821-24 and PSR 1937+21, might be prone to rotation irregularities while old millisecond pulsars like PSR 1855+09 are 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 disk and bulge. Finally, we present the daily timing observations of PSR 1821-24 conducted every year since 1989 when the solar corona intervenes between the pulsar and the Earth at Christmas time. We have used these timing observations along a single cut through the corona to fit a spherically symmetric model of its electron density n_e=n₀x(r/r₀)^–α. With a slightly more sophisticated model for the coronal electron density, precise timing observations of PSR 1821-24 during these periods over a complete solar cycle could monitor the global flattening expected for the coronal plasma at the solar minimum.