The nature of the accretion physics in quiescent black hole system LB-1

TL;DR

This paper investigates the accretion physics in the LB-1 black hole system by modeling the accretion flow and comparing predicted spectra and luminosity with observations, constraining the viscosity parameter within the disk evaporation framework.

Contribution

It provides a detailed accretion flow model for LB-1, linking observed spectral features to accretion physics and constraining the viscosity parameter based on X-ray luminosity limits.

Findings

Predicted truncation radius matches observed broad Hα emission.

X-ray luminosity predicted is below Chandra's detection threshold.

Constraints on the viscosity parameter α based on observed X-ray limits.

Abstract

LB-1 is a binary system that has drawn great attention since its discovery in 2019. The nature of the two components of LB-1 is not very clear, which however is suggested very possibly 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$_\alpha$ emission line. The predicted X-ray luminosity is definitely below the estimated upper limits with the sensitivity of Chandra X-ray Observatory of the X-ray luminosity $\sim 2\times 10^{31}$ erg/s. Finally, we argue that if the disk evaporation model indeed reflects the intrinsic physics of the accretion flow, the value of the viscosity parameter $\alpha$ is constrained to be $\alpha \gtrsim 0.05$ (with BH mass being $68M_{\rm \odot}$), or $\alpha \gtrsim 0.003$ (with BH mass being $21M_{\rm \odot}$) to match the observed upper limit of the X-ray luminosity of LB-1.