Extending the Shakura-Sunyaev approach to a strongly magnetized accretion disc model

Abstract

We develop a model of thin turbulent accretion discs supported by magnetic pressure of turbulent magnetic fields. This applies when the turbulent kinetic and magnetic energy densities are greater than the thermal energy density in the disc. Whether such discs survive in nature or not remains to be determined, but here we simply demonstrate that self-consistent solutions exist when the α-prescription for the viscous stress, similar to that of the original Shakura-Sunyaev model, is used. We show that α ∼ 1 for the strongly magnetized case and we calculate the radial structure and emission spectra from the disc in the regime when it is optically thick. Strongly magnetized optically thick discs can apply to the full range of disc radii for objects ≤ 10-2 of the Eddington luminosity or for the outer parts of discs in higher luminosity sources. In the limit that the magnetic pressure is equal to the thermal or radiation pressure, our strongly magnetized disc model transforms into the Shakura-Sunyaev model with α = 1. Our model produces spectra quite similar to those of standard Shakura-Sunyaev models. In our comparative study, we also discovered a small discrepancy in the spectral calculations of Shakura & Sunyaev (1973).