Simultaneous ultraviolet, optical, and X-ray observations of the X-ray source Vela X-1 (HD 77581)

Ultraviolet spectra of HD 77581, associated with the binary X-ray source Vela X-1, taken with the International Ultraviolet Explorer satellite (IUE) show a spectrum typical of an early B-type supergiant. However, the P Cygni profiles of strong resonance lines show substantial variations with orbital phase. These variations can be ascribed to the changing ionization state in the stellar wind caused by the X-ray emitting companion as suggested by Hatchett and McCray. The mass loss of the supergiant primary is determined to be approx.1 x 10-6 M/sub sun/ yr-1. X-ray and spectroscopic and photometric optical observations, simultaneous with the IUE measurements, indicate behavior consistent with previous epochs. The interstellar spectrum shows strong, relatively broad lines of highly ionized Si IV and CIV which may result from the effects of X-rays upon the interstellar material neighboring the source