Scattering of X-rays on the winds of supergiant X-ray binaries.

Abstract

Monte-Carlo simulations of Thomson scattering of X-rays in the stellar wind of OB stars in supergiant X-ray binaries show that at least part of the low-energy excess seen in the X-ray spectra during phases of high absorption can be explained by scattering of X-rays around a dense region in the wind obscuring the direct radiation. In the case of 4U 1700-37 and Vela X-1 the fraction of scattered X-rays derived from EXOSAT ME spectra increases toward phase 0.5. This is consistent with the model in which the majority of X-rays is scattered in the dense innermost regions of the wind and the stellar surface of the early-type star. At phase 0.5 the full part of the stellar surface illuminated by X-rays is visible to the observer. During eclipse of the X-ray source the X-ray flux consists of X-rays scattered in the wind, and the contribution of a gas stream with higher wind density trailing the neutron star results in an asymmetric eclipse light curve with a higher X-ray intensity at eclipse ingress compared to egress as seen from other supergiant X-ray binaries.