Hydrodynamical winds from two-temperature plasma in X-ray binaries

Hydrodynamical winds from a spherical two-temperature plasma surrounding a compact object are constructed. The mass-loss rate is computed as a functio n of electron temperature, optical depth and luminosity of the sphere, the values of which can be constrained by the fitting of the spectral energy di stributions for known X-ray binary systems. The sensitive dependence of the mass-loss rate with these parameters leads to the identification of two di stinct regions in the parameter space separating wind-dominated from non-wi nd-dominated systems. A critical optical depth (tau (c)) is defined as a fu nction of luminosity and electron temperature, which differentiates these t wo regions. Systems with optical depths significantly smaller than tau (c) are wind-dominated. The results are applied to black hole candidate X-ray binary systems in the hard spectral state (Cyg X-1, GX 339-4 and Nova Muscae), and it is found t hat the inferred optical depth (tau) is similar to tau (c), suggesting that they are wind-regulated systems. On the other hand, for X-ray binary syste ms containing a neutron star (e.g., Cyg X-2) tau is much larger than tau (c ) indicating the absence of significant hydrodynamical winds.