X-ray and optical observations of 3U 0900 - 40 (Vela X-1)

Copernicus and Ariel V observations of 3U 0900 – 40 are presented. During 1975 February a region of low intensity was observed between binary phase 0.5 and eclipse which was caused by an increase in photoelectric absorption. This low state can be interpreted as the signature of an accretion wake travelling across the line of sight. The source also showed energy-independent flaring behaviour on timescales of approximately 1 hr. The 283-s period measured by Copernicus in 1975 February was 0.017 s longer than that observed by SAS-3 in 1975 July. This is consistent with the secular decrease in period expected for an accreting neutron star. An upper limit of 0.004 mag is placed on the amplitude of broadband 283-s optical pulsations.