LINE-DOMINATED ECLIPSE SPECTRUM OF VELA X-1

The eclipsing X-ray binary pulsar Vela X-1 (4U 0900-40) was observed with ASCA twice in 1993 June and July covering the total eclipse transition from ingress to egress by the two observations. The pre-eclipse and posteclipse spectra are composed of a heavily absorbed pulsed component and a flat soft component which is ascribed to scattering by the stellar wind. In contrast, only a nonpulsating scattering component is visible during eclipse, and this spectrum is remarkably line dominated with a flat continuum. In addition to an intense 6.4 keV emission line, which is due to fluorescence of iron in low ionization states, prominent line features are obtained at 0.92, 1.34, and 1.82 keV. These lines are identified as the Ka lines of He-like ions of neon, magnesium, and silicon. Significant evidence of He-like Ka lines is also obtained for sulfur, argon, and iron. An eclipse spectrum dominated by He-like Ka emission lines of heavy elements suggests that radiative recombination followed by cascades is the dominant process in the X-ray-irradiated stellar wind, since such a stellar wind forms highly ionized zones of He-like and H-like ions with a relatively low electron temperature of about 100 eV.