Accretion with two-phase gas supply and its application in black hole X-ray binaries

TL;DR

This paper explores a combined cold and hot gas supply model for black hole X-ray binaries, analyzing how different gas ratios influence accretion geometry and spectral states, and compares predictions with observations of Cygnus X-1.

Contribution

It introduces a semi-analytical model of accretion with dual gas supplies, revealing how gas ratios affect accretion structure and spectral states, and constrains the viscosity parameter from observations.

Findings

Accretion geometry varies with gas supply rate and cold/hot gas ratio.

Inner flow becomes disc-dominated at higher accretion rates, leading to soft states.

Predicted hardness-intensity correlation matches Cygnus X-1 data.

Abstract

Accretion in black hole X-ray binaries is commonly believed to be supplied by the Roche lobe overflow or the stellar wind. The former is thought to form a geometrically thin disc while the diffuse wind could form a geometrically thick hot accretion flow. In this paper, we instead consider a more generalised case, i.e., accretion with both cold and hot gas supplies, which feed a disc and a corona respectively. We investigate the interaction of disc and corona by analysing the energy coupling and matter exchange, i.e. corona condensation/disc evaporation, with a semi-analytical method. It is found that the accretion geometry in the radial direction and the resultant emission spectrum depend strongly on both the total gas supply rate and the ratio of cold and hot gases. For gas supply rates of a few percent of the Eddington value, diverse geometries and spectral shapes are possible, depending on the fraction of cold gas supply. This provides an interpretation for the various spectra observed in intermediate states. However, at higher accretion rates, regardless of the form of the feeding gas, the inner accretion flow is always disc-dominated, implying an inevitable transition to the soft state, while at very low gas supply rates, hard state spectrum dominated by the hot flow is expected. We also present the predicted hardness-intensity correlation of Cygnus X-1, and constrain the value of the viscosity parameter of the accretion flow to the range of 0.25--0.35 by comparing our results with MAXI observations.