X-RAY-SCATTERING AND FLUORESCENCE IN THE WIND OF A MASSIVE X-RAY BINARY

A number of massive X-ray binaries exhibit a small (few percent) residual X-ray intensity during eclipse. This radiation is presumed to result from scattering and fluorescence in the stellar wind and atmosphere of the massive companion star. In addition, a number of these same sources exhibit a soft X-ray "excess" in their spectra at other orbital phases, an effect which we show is due to the same physical processes. In this work we utilize orbital-phase-resolved X-ray spectra from the X-ray binary 4U 0900-40 (Vela X-1), obtained with the Ginga satellite, to study the distribution of matter in the binary system. The Ginga spectra are interpreted with the help of simulated spectra that are generated with a Monte Carlo scattering code. The scattering code and the simulations are described in some detail. We find that scattering and fluorescence in a simple spherically symmetric distribution of matter surrounding the companion star is able to reproduce the eclipse spectrum as well as the "soft X-ray excess" observed during egress and other orbital phases. We find reasonably secure values for a number of the parameters that characterize the density profile of the stellar wind and atmosphere of the companion star. The Fe abundance is within a factor of approximately 1.3 of that in the solar neighborhood. We show that ionization zones are not critical to understanding the orbital-phase-resolved spectra (in the 2-35 keV energy band observed with Ginga) in this relatively low-luminosity source. We also evaluate the contribution to the observed spectra during eclipse from scattering by interstellar dust grains and from the diffuse emission from the "Galactic ridge." We conclude that the contribution from interstellar grains is negligible, while that from the Galactic ridge may be as high as approximately 25% in the energy channels below approximately 4 keV.