The Nature of the Accretion Physics in Quiescent Black Hole System LB-1

Abstract LB-1 is a binary system that has drawn great attention since its discovery in 2019. The two components of LB-1 are suggested to be a B-type star plus a black hole (BH). In this paper, we first calculate the wind mass-loss rate of the B-type star. We then calculate the mass capture rate by the BH, with which as the initial mass accretion rate, we calculate the truncation radius of the accretion disk and the corresponding emergent spectra of the accretion flow (comprising an inner advection-dominated accretion flow (ADAF) + an outer truncated accretion disk) within the framework of the disk evaporation model. It is found that the predicted truncation radius of the accretion disk with appropriate model parameters is consistent with observations inferred from the observed broad H α emission line. The predicted X-ray luminosity is definitely below the estimated upper limits with the sensitivity of the Chandra X-ray Observatory of the X-ray luminosity ≈2 × 10 31 erg s −1 , which is also consistent with observations. Finally, we argue that the disk evaporation model can indeed reflect the intrinsic physics of the accretion flow in LB-1, and has potential to be applied to other quiescent BHs in the future.