Observations of stellar winds in high-mass X-ray binaries : evidence for a non-monotonic velocity structure.

Abstract

We report variations with orbital phase in UV resonance lines of HD77581 (B0.5 Iab), the optical counterpart of the high-mass X-ray binary system Vela X-1 (4U0900-40), occurring at low and intermediate negative velocities. We argue that these variations are caused by ionization effects in the stellar wind by the X-ray source, just like the well-known orbital modulation of the high-velocity part of the P Cygni absorption, as was predicted by Hatchett & McCray (1977). We also present UV observations of the C III subordinate line at 2296 A of HD 153919 (O6.5 Iaf(+)), the optical counterpart of 4U1700-37, which support our earlier interpretation that the orbital variations observed in the subordinate He II 1640 A and N IV 1718 A lines in this star are the result of variable Raman-scattered emission lines. We show that it is difficult to give a consistent explanation of the observed orbital variations in UV spectra of HD77581, if we adopt a monotonic velocity law for the stellar wind. Both the orbital variations at low and high velocities in HD77581 and the absence of similar changes in UV resonance lines of HD153919 can be better understood if the velocity structure of the supergiant's wind is non-monotonic. Such a velocity structure is predicted by recent hydrodynamical calculations of radiation-driven winds. The clumpiness of a shocked stellar wind can also explain the observed soft X-ray excess and the flaring in X-ray luminosity, caused by local changes in the mass-accretion rate.