Disk-Evaporation Fed Corona: Structure and Evaporation Feature with Magnetic Field

TL;DR

This study incorporates magnetic fields into the disk-corona evaporation model for black hole X-ray binaries, revealing their influence on coronal structure, evaporation rates, and spectral state transition radii, aligning better with observations.

Contribution

It explicitly accounts for magnetic fields in the evaporation model, showing their effects on coronal structure and state transition radii, improving agreement with observations.

Findings

Maximal evaporation rate remains roughly constant at 0.03 Eddington rate.

Increasing magnetic field strength decreases the radius of maximal evaporation.

Spectral state transition occurs at a few percent of Eddington accretion rate.

Abstract

The disk-corona evaporation model naturally interprets many observational phenomena in black hole X-ray binaries, such as the truncation of an accretion disk and the spectral state transitions. On the other hand, magnetic field is known to play an important role in transporting angular momentum and producing viscosity in accretion flows. In this work, we explicitly take the magnetic field in the accretion disk corona into account and numerically calculate the coronal structure on the basis of our two-temperature evaporation code. We show that the magnetic field influences the coronal structure by its contribution to the pressure, energy and radiative cooling in the corona and by decreasing the vertical heat conduction. We found that the maximal evaporation rate keeps more or less constant ($\sim 0.03$ Eddington rate) while the strength of magnetic fields changes, but that the radius corresponding to the maximal evaporation rate decreases with increasing magnetic field. This predicts that the spectral state transition always occurs at a few percent of Eddington accretion rate, while the inner edge of thin disk can be at $\sim 100 R_{\rm S} $ or even less in the hard state before the transition to the soft state. These results alleviate the problem that previous evaporation models predict too large a truncation radius, and are in better agreement with the observational results of several black hole X-ray binaries, though discrepancies remain.