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M. Sako (Columbia Astrophysics Laboratory), D. A. Liedahl (Lawrence Livermore National Laboratory), S. M. Kahn (Columbia Astrophysics Laboratory), F. Paerels (Space Research Organization of the Netherlands)
The ASCA eclipse spectrum of Vela X-1 exhibits two distinct sets of discrete features: (1) recombination lines and radiative recombination continua from mostly hydrogenic and helium-like species produced by photoionization in an extended stellar wind; and (2) fluorescent K-shell lines associated with near-neutral species also present in the circumsource medium. The presence of emission lines from these two extreme degrees of ionization states cannot be explained by emission originating from a smooth stellar wind alone. Through a detailed differential emission measure analysis of the Vela X-1 spectrum, we find that a model in which hundreds of cool clumps are embedded in a hotter, more ionized gas can account for the observed line spectrum. Most of the wind mass is contained in the clumps, while most of the wind volume (>95%) is occupied by the highly ionized component.
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