Masses of the visual components and black holes in X-ray novae: Effects of proximity of the components

Abstract

It is shown that the approximation of the complex, tidally distorted shape of a star as a circular disc with local line profiles and a linear limb-darkening law, which is usually applied when deriving equatorial stellar rotation velocities from line profiles, leads to overestimation of the equatorial velocity V_rot sin i and underestimation of the component mass ratio q = M_x/M_v. A formula enabling correction of the effect of these simplifying assumptions on the shape of a star is used to re-determine the mass ratios q and the masses of the black holes M_x and visual components M_v in low-mass X-ray binary systems containing black holes. Taking into account the tidal–rotational distortion of the stellar shape can significantly increase the mass ratios q = M_x/M_v, reducing M_v, while M_x changes only slightly. The resulting distribution of M_v attains its maximum near M_v ≃ 0.35M_⊙, in disagreement with the results of population synthesis computations realizing standard models for Galactic X-ray novae with black holes. Possible ways to overcome this inconsistency are discussed. The derived distribution of M_x also differs strongly from the mass distribution for massive stars in the Galaxy.