Clumpy-gas radiation hydrodynamics: Application to supersoft X-Ray sources

Abstract Supersoft X-ray sources (SSXSs) sometimes exhibit an anti-correlation between an X-ray flux and an optical one. This curious nature can be explained by a clumpy wind from a luminous disk surrounding a luminous central white dwarf under the clumpy-gas radiation hydrodynamical (CRHD) framework. That is, the efficiency of the radiative force acting on cloudlets with finite optical depths is less than unity, compared with that on the ambient gas. Hence, the escape conditions for cloudlets and gas are different, and depend on luminosities of the central star, $L_{\rm X}$, which are of the order of the Eddington one, $L_{\rm E}$. When the bolometric (X-ray) luminosity is relatively small, as $L_{\rm X} \sim 0.5~L_{\rm E}$, only the tenuous gas can blow off from the luminous disk (the disk sky is clear) and the optical emission from the disk can pass through the disk wind to reach a distant observer, making an optically high state. When $L_{\rm X} \sim L_{\rm E}$, cloudlets with optical depths of the order of unity can be also ejected from the disk (the disk sky is cloudy), and the disk optical emission would be obscured, making an optically low state. This property is mainly determined by energetics of outflows from a rotating disk around a luminous source, and therefore, the results may be rather universal in SSXSs.