Appendix A: X-ray eclipse spectra

Averages of spectra near $\phi =0$ (eclipse of the X-ray source) are a fair representation of the "undisturbed'' stellar wind. The low-resolution spectra of HDE 226868/Cyg X-1, Sk 160/SMC X-1, and Sk-Ph/LMC X-4 all contain the strong geocoronal Lyman-$\alpha$ emission line at 1215.34 Å, and the resonance lines of C  IV near 1550 Å and Si  IV near 1400 Å (Fig. 1). These are much stronger in HDE 226868/Cyg X-1 than in the two Magellanic Cloud members that both have a lower metalicity than their counterparts in the Milky Way.

Also immediately clear is the difference between the continua of HDE 226868/Cyg X-1 and the two Magellanic Cloud members. Due to the high extinction towards HDE 226868/Cyg X-1, its continuum is tilted both at the 1200 Å side (dust and hydrogen) and at the 2100 Å side (grains or large molecules). The Magellanic Cloud members are much less affected by interstellar extinction.

Figure A.1: X-ray eclipse spectra of HDE 226868/Cyg X-1, Sk 160/SMC X-1, and Sk-Ph/LMC X-4.

The high resolution spectra have not been flux calibrated in an absolute sense, therefore the shape of the continuum in Figs. A2 and A3 does not represent the shape of the continuum as it entered the telescope.

The geocoronal Ly-$\alpha$ line and the resonance lines of N  V, Si  IV, C  IV and Al  III are the most prominent features in the spectrum of HD 77581/Vela X-1 (Fig. A.2, top). N  V and Al  III are very strong for its spectral type. Notice the rich Fe  III spectrum at wavelengths longer than $\sim$1880 Å, the Fe  IV spectrum between $\sim$1480 Å and 1880 Å, and the lack of Fe  V and Fe  VI at wavelengths  Å.

In the spectrum of HD 153919/4U1700-37 (Fig. A.2, bottom) the resonance lines of N  V, Si  IV and C  IV and the subordinate line of N  IV at 1718.551 Å are very strong. The resonance line of Al  III is mainly interstellar. Notice that the Fe  III and Fe  IV spectra are absent, whilst at wavelengths  Å the spectrum is crowded with lines of Fe  V and Fe  VI. This contrast with HD 77581/Vela X-1 is due to the higher photospheric temperature of the O6.5 Iaf⁺ star HD 153919 compared to the B0.5 Iab star HD 77581. The stronger P-Cygni lines of Si  IV and C  IV in HD 153919/4U1700-37 reflect the denser and faster stellar wind of HD 153919 compared to HD 77581. Note that the N  V emission is stronger in HD 77581/Vela X-1.

Figure A.2: X-ray eclipse spectra of HD 77581/Vela X-1 (top) and HD 153919/4U1700-37 (bottom).