1994A&A...289..846K

We present high-resolution, high signal-to-noise spectra of HD77581 and HD153919, the optical counterparts of the high-mass X-ray binaries Vela X-1 and 4U 1700-37, respectively. The spectral lines exhibit variations related to the presence of the X-ray source in unprecedented detail. For HD77581, at inferior conjunction of the neutron star (φ=0.5), an extra absorption component appears in the blue wing of the absorption lines. This feature first increases in strength (up till binary phase φ=0.6) whereafter it slowly fades; at the same time it shifts towards higher negative velocities. The observed velocities are low: from -50km/s at φ=0.5 up to -250km/s at φ=0.8. For HD153919, the observed variations are similar in character, but stronger and more complicated. For both systems we find significant variations in the emission part of the P Cygni-type profiles. Based on the phase dependence of the velocity and strength of the absorption component, we show that it is unlikely that this component is related to an accretion wake, or to a gas stream through the inner Lagrangean point. These structures, although most likely present in the system, are not capable of providing sufficient obscuration of the supergiant, that would give the observed absorption. We suggest that this absorption component results from the presence of a photo-ionization wake in the system that trails the X-ray source. Furthermore, we predict that variations in X-ray luminosity will induce changes in the geometrical structure of the photo-ionization wake which might explain the observed orbit-to-orbit variations.