Disc accretion onto a binary black hole in a hierarchical triple system as an origin of the most luminous hyper-soft sources

We propose that the recently discovered luminous hypersoft X-ray sources can be explained by accretion onto a binary black hole in a hierarchical triple system. For black hole masses $\sim 15M_{\odot }$, the orbital separation of the internal binary might be $\sim 0.01$~AU. If the donor provides $\gtrsim {10}^{-8}{M}_{\odot }$ yr${}^{-1}$, then the circumbinary accretion disc can explain the observed properties of the most luminous supersoft sources.