The X-Ray Spectrum and Global Structure of the Stellar Wind in Vela X-1

We present a quantitative analysis of the X-ray spectrum of the eclipsing high mass X-ray binary Vela X-1 (4U 0900-40) using archival data from the ASCA Solid-State Imaging Spectrometer. The observation covers a time interval centered on the eclipse of the X-ray pulsar by the companion. The spectrum 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. These features are superposed on a faint continuum, which is most likely nonthermal emission from the accreting neutron star that is scattered into our line of sight by free electrons in the wind. Using a detailed spectral model that explicitly accounts for the recombination cascade kinetics for each of the constituent charge states, we are able to obtain a statistically acceptable (χ = 0.88) fit to the observed spectrum and to derive emission measures associated with the individual K-shell ions of several elements. From calculations of the ionization balance using the photoionization code, XSTAR, we assign ionization parameters, ξ, to several ions, and construct a differential emission measure (DEM) distribution. The DEM distribution spans a broad range in ξ (Δ log ξ ≳ 2) and is centered around log ξ = 2.5. We find that the total emission measure of the visible portion of the highly ionized wind is ~3 × 10⁵⁶ cm^-3. The qualitative aspects of the inferred DEM distribution are consistent with a wind model derived from the Hatchett & McCray picture of an X-ray source immersed in a stellar wind with a generalized Castor, Abbott, & Klein velocity profile. Using this formalism, theoretical DEM distributions, parameterized only by a mass-loss rate and a wind velocity profile, are calculated and used to predict the detailed X-ray spectrum, which is then compared to the ASCA data. Again, we find a statistically acceptable fit (χ = 1.01), with a best-fit mass-loss rate of ~2.7 × 10^-7 M_☉ yr^-1. This is approximately a factor of 10 lower than previous estimates of the mass-loss rate for the Vela X-1 companion star, which have primarily been determined from C IV and Si IV P Cygni profiles, and X-ray absorption measurements. We argue that this discrepancy can be reconciled if the X-ray-irradiated portion of the wind in Vela X-1 is structurally inhomogeneous, consisting of hundreds of cool, dense clumps embedded in a hotter, more ionized gas. Most of the mass is contained in the clumps, while most of the wind volume (>95%) is occupied by the highly ionized component. We show quantitatively that this interpretation is also consistent with the presence of the X-ray fluorescent lines in the ASCA spectrum, which are produced in the cooler, clumped component.