The orbital parameters and the X-ray pulsation of VELA X-1 (4U 0900-40).

Results are presented of an analysis of 83 d of 2 - 12 keV X-ray observations of Vela X-1 (4U 0900-40) obtained with the ESA COS-B satellite. Combining the present determinations of the orbital epoch from pulse timing with those obtained with other satellites, the data are found to be consistent with an increase of the orbital period at a rate (P/P)_orb = (2.1±0.6)×10^-5yr^-1 during the interval 1975 - 1982. This result is discussed in terms of models for orbital-period changes. The pulsation period is shown to undergo very rapid intrinsic changes, at a rate of up to P/P ≡ 10^-1yr^-1 during intervals of a few days. It is argued that transfer of angular momentum to the neutron star by accreting matter or by means of the propeller effect is very unlikely to be sufficient to explain these pulsation-period changes. It is suggested that short-term fluctuations in the pulse-arrival times may occur which are due to shifts in the direction in which the pulse is emitted.