Modelling the orbital modulation of ultraviolet resonance lines in high-mass X-ray binaries

Abstract

The stellar-wind structure in high-mass X-ray binaries (HMXBs) is investigated through modelling of their ultraviolet (UV) resonance lines. For the OB supergiants in two systems, Vela X-1 and 4U1700 37, high- resolution UV spectra are available; for Cyg X-1, SMC X-1, and LMC X-4 low-resolution spectra are used. In order to account for the non-monotonic velocity structure of the stellar wind, a modied version of the Sobolev Exact Integration (SEI) method by Lamers et al. (1987) is applied. The orbital modulation of the UV resonance lines provides information on the size of the Stromgren zone surrounding the X-ray source. The amplitude of the observed orbital modulation (known as the Hatchett-McCray eect), however, also depends on the density- and velocity structure of the ambient wind. Model proles are presented that illustrate the eect on the appearance of the HM eect by varying stellar-wind parameters. The q parameter of Hatchett & McCray (1977), as well as other parameters describing the supergiant's wind structure, are derived for the 5 systems. The X-ray luminosity needed to create the observed size of the Stromgren zone is consistent with the observed X-ray flux. The derived wind parameters are compared to those determined in single OB supergiants of similar spectral type. Our models naturally explain the observed absence of the HM eect in 4U1700 37. The orbital modulation in Vela X-1 indicates that besides the Stromgren zone other structures are present in the stellar wind (such as a photo- ionization wake). The ratio of the wind velocity and the escape velocity is found to be lower in OB supergiants in HMXBs than in single OB supergiants of the same eective temperature.