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M. Sako, S. M. Kahn, F. Paerels (Columbia University), D. A. Liedahl, P. S. Wodjowski (LLNL), F. Nagase (ISAS), B. Paul (Tata Institute of Fundamental Research), K. Makishima (University of Tokyo), N. Schulz (MIT)
We present preliminary results from Chandra HETG observations of the archetypal wind-fed high-mass X-ray binary Vela X-1. The observations covered three orbital phase ranges, centered on \phi = 0.00, 0.25, and 0.50. The spectra show dramatic changes with orbital phase. During X-ray eclipse (\phi ~0.0), the spectrum is completely dominated by line emission superimposed on a weak continuum, which is scattered presumably from the same medium that produces the line radiation. The observed spectrum during \phi ~0.25, on the other hand, consists primarily of continuum radiation from the neutron star with several weak resonance absorption lines. At \phi ~0.5, the long-wavelength region of the spectrum is, again, dominated by line emission, with intensities higher by an order of magnitude compared to those during eclipse. Bulk velocity fields of ~500 ~\rm{km~s}-1 are detected in the various lines. We discuss the implications of our results in relation to the global properties of the stellar wind.