2007ApJ...668.1145Q

The disk-corona evaporation model naturally accounts for 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 fields are 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 consideration 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 through its contribution to the pressure, energy, and radiative cooling in the corona and by decreasing the vertical heat conduction. We find that the maximum evaporation rate stays more or less constant (∼3% of the Eddington rate) when the strength of the magnetic field changes, but that the radius corresponding to the maximum evaporation rate decreases with increasing magnetic field. This suggests that spectral state transitions always occur at a few percent of the Eddington accretion rate, while the inner edge of the thin disk can be at ∼100R_S or even less in the hard state before the transition to the soft state. These results alleviate the problem of previous evaporation models' predicting too large a truncation radius and are in better agreement with the observational results for several black hole X-ray binaries, although discrepancies remain.