6 Summary

An analysis is presented of a large set of IUE spectra for five HMXBs: HDE 226868/Cyg X-1, Sk-Ph/LMC X-4, Sk 160/SMC X-1, HD 77581/Vela X-1 and HD 153919/4U1700-37. We compared their spectra and variability, and adapted the SEI radiation transfer code for modelling the variability of resonance lines in HMXBs. From model fits for HD 77581/Vela X-1 and HD 153919/4U1700-37 we derive terminal velocities and ionization fractions of their stellar winds, as well as the sizes of the Strömgren zone created in the wind by the X-ray source. For three other HMXBs (HDE 226868/Cyg X-1, Sk-Ph/LMC X-4 and Sk 160/SMC X-1) only rough estimates are derived for the terminal velocity and size of the Strömgren zone.

The adapted SEI model reproduces the Si  IV, C  IV and Al  III resonance line profiles and their orbital modulation in HD 77581/Vela X-1, with additional absorption components attributed to a photo-ionization wake. The N  V resonance line in HD 77581/Vela X-1 is more difficult to model, and it seems to be severely affected by the photo-ionization wake. The N  V, Si  IV and C  IV line profiles in HD 153919/4U1700-37 are well reproduced by the adapted SEI model; their lack of orbital modulation is due to the dense stellar wind and low X-ray luminosity.

OB supergiants in HMXBs have lower terminal velocities than single stars of similar spectral type, possibly due to a lower effective gravity of the Roche-Lobe filling primary. Ionization fractions in the stellar wind of HD 77581/Vela X-1 agree with predictions for single stars, except for an overabundance of N⁴⁺ ions in the wind of HD 77581. This may be due to super-ionization possibly resulting from X-ray emission from shocks at the base of the wind, or nitrogen overabundance due to mass-transfer from the progenitor of Vela X-1 or extensive mass loss from HD 77581 itself. The N⁴⁺ abundance in the stellar wind of HD 153919/4U1700-37 agrees very well with predictions for single. However, the ionization fractions of the other ions (Si³⁺ and C³⁺) are much lower than predicted, suggesting a generally lower ionization of the stellar wind of HD 153919 than what is predicted for single stars. This would then infer an N⁴⁺ over-abundance in the stellar wind of HD 153919. The sizes of the Strömgren zones are in fair agreement with the expectations from standard Bondi & Hoyle accretion, and they are larger for more luminous X-ray sources.

Acknowledgements

We thank Henny Lamers for interesting discussions and for supplying the original SEI code, and an anonymous referee for her/his remarks. LK is supported by a fellowship of the Royal Academy of Sciences in The Netherlands. Much of the work presented here was carried out while JvL was in Amsterdam for his MSc. Mas este trabalho nunca podia estar feito sem o encargo pelo anjo Joana.