Hydrodynamic simulations of stellar wind disruption by a compact X-ray source

We present two-dimensional numerical simulations of the gas flow in the orbital plane of a massive X-ray binary system, in which the mass accretion if fueled by a radiation-driven wind from an early-type companion star. We use these simulations to examine the role of the compact object (either a neutron star or a black hole) in disturbing the radiatively accelerating wind of the OB companion, with an emphasis on understanding the origin of the observed soft X-ray photoelectric absorption seen at late orbital phases in these systems. The flow of the stellar wind past the compact star is influenced by a variety of competing effects, including gravitational, rotational, and radiation pressure forces and X-ray heating. Our simulations are characterized by nonsteady accretion wakes at moderate X-ray luminosities (∼10³⁶ ergs s-1) producing filaments in the downstream wake with densities approaching ∼100 times that of the undisturbed wind. At high X-ray luminosities (≥10³⁷ ergs s^-1), Compton cooling at small radii results in a steady radial accretion flow onto the compact object. The presence of a photoionization wake, as suggested by Fransson and Fabian, does not contribute significantly to the integrated column density until the wind is X-ray-ionized most of the way to the surface of the companion OB star (requiring high X-ray luminosities and low wind densities). On the basis of these simulations, we suggest that the phase-dependent photoelectric absorption seen in several of these systems can be explained by dense filaments of compressed gas formed in the nonsteady accretion bow shock and wake of the compact object.