Bimodality of wind-fed accretion in high-mass X-ray binaries

We here consider the influence of X-ray photoionization from an accreting neutron star in a high-mass X-ray binary. Our aim is to unveil a new principle governing the X-ray luminosity of X-ray binaries, through a simple analysis of fluid equations simulating line-driven wind flow under the influence of X-ray irradiation. In this study, we solved the equation of motion of the accretion flow while taking into account the line-driven acceleration and X-ray photoionization. Under the influence of X-ray irradiation, we have found that the flow equations have two types of solutions. The first is characterized by a slow wind velocity, which causes a large accretion rate. The second solution is a fast wind flow, which results in a small accretion rate. We find that only the solution with a fast wind and faint X-ray luminosity is a steady solution. On the other hand, a slow wind solution with a large X-ray luminosity is not a realizable solution. In bright X-ray binary systems, the X-ray luminosity would increase until strong X-rays reduce the line-driven acceleration, and cause a stagnation of the wind. This implies an important consequence; that is, the X-ray luminosity of the wind-fed, X-ray emitting binary is settled by the limit of wind stagnation. At the same time, the fast-wind solution with a small X-ray luminosity can also represent a steady state. Bright X-ray sources, such as Vela X-1, would have limiting luminosities of wind stagnation, while faint systems, such as quiescent supergiant fast X-ray transients, could follow the faint solution.