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

The stellar-wind structure in high-mass X-ray binaries (HMXBs) is investiga ted through modelling of their ultraviolet (UV) resonance lines. For the OB supergiants in two systems, Vela X-1 and 4U1700-37, high-resolution UV spe ctra are available; for Cyg X-1, SMC X-1, and LMC X-4 low-resolution spectr a are used. In order to account for the non-monotonic velocity structure of the stellar wind, a modified 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 s urrounding the X-ray source. The amplitude of the observed orbital modulati on (known as the Hatchett-McCray effect), however, also depends on the dens ity- and velocity structure of the ambient wind. Model profiles are present ed that illustrate the effect on the appearance of the HM effect by varying stellar-wind parameters. The q parameter of Hatchett & McCray (1977), as w ell as other parameters describing the supergiant's wind structure, are der ived 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 super giants of similar spectral type. Our models naturally explain the observed absence of the HM effect in 4U1700-37. The orbital modulation in Vela X-1 i ndicates that besides the Stromgren zone other structures are present in th e stellar wind (such as a photoionization wake). The ratio of the wind velo city and the escape velocity is found to be lower in OB supergiants in HMXB s than in single OB supergiants of the same effective temperature.