Pulsations in the ultraviolet and X-ray spectra of Vela X-1

In simultaneous observations with the Faint Object Spectrograph aboard the Hubble Space Telescope and the large-area counters aboard Ginga, we obtained time-resolved ultraviolet and X-ray spectra of the X-ray binary HD 77581/4U 0900 - 40 (Vela X-1) during inferior conjunction of the X-ray source. We detected pulsations at the 283.33 s X-ray pulse period in the absorption lines Si IV lambda 1402 and N V lambda lambda 1238, 1242. The pulse amplitudes in the UV lines were similar to 3%. The profile of the pulsed UV intensity in the Si IV absorption line resembled that of the soft X-rays, while that in the N V line was anticorrelated with the profile of the hard X-ray pulsations. The effect is probably due to photoionization of Si IV and N IV in the stellar wind of HD 77581 by the rotating beam of Vela X-1.