X-ray irradiation of the stellar wind in HMXBs with B supergiants: Implications for ULXs

Abstract

Wind-fed high-mass X-ray binaries are powered by accretion of the radiatively driven wind of the luminous component on the compact star. Accretion-generated X-rays alter the ionization state of the wind. Because higher ionization states drive the wind less effectively, X-ray ionization may brake acceleration of the wind. This causes a decrease in the wind terminal velocity and mass flux in the direction toward the X-ray source. Here we study the effect of X-ray ionization on the stellar wind of B supergiants. We determine the binary parameters for which the X-ray irradiation significantly influences the stellar wind. This can be conveniently studied in diagrams that plot the optical depth parameter versus the X-ray luminosity. For low optical depths or for high X-ray luminosities, X-ray ionization leads to a disruption in the wind aimed toward the X-ray source. Observational parameters of high-mass X-ray binaries with B-supergiant components appear outside the wind disruption zone. The X-ray feedback determines the resulting X-ray luminosity. We recognize two states with a different level of feedback. For low X-ray luminosities, ionization is weak, and the wind is not disrupted by X-rays and flows at large velocities, consequently the accretion rate is relatively low. On the other hand, for high X-ray luminosities, the X-ray ionization disrupts the flow braking the acceleration, the wind velocity is low, and the accretion rate becomes high. These effects determine the X-ray luminosity of individual binaries. Accounting for the X-ray feedback, estimated X-ray luminosities reasonably agree with observational values. We study the effect of small-scale wind inhomogeneities (clumping), showing that clumping weakens the effect of X-ray ionization by increasing recombination and the mass-loss rate. This effect is particularly important in the region of the so-called bistability jump. We show that ultraluminous X-ray binaries with LX ≲ 1040 erg s−1 may be powered by accretion of a B-supergiant wind on a massive black hole.