2013A&A...554A..37D

The stellar wind around a compact object in luminous wind-accreting high-mass X-ray binaries is expected to be strongly ionized with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionization significantly reduces the radiative force within the so-called Stroemgren region, leading to wind stagnation around the compact object. In close binaries like Vela X-1 this effect might alter the wind structure throughout the system. Using the spectral data from the Monitor of All-sky X-ray Image (MAXI), we studied the observed dependence of the photoelectric absorption as a function of the orbital phase in Vela X-1 and found that it is inconsistent with expectations for a spherically symmetric smooth wind. Taking into account previous investigations, we developed a simple model for wind structure with a stream-like photoionization wake region of slower and denser wind trailing the neutron star, which is responsible for the observed absorption curve.