Global Structure of Optically Thin, Magnetically Supported, Two-Temperature, Black Hole Accretion Disks

TL;DR

This paper develops global models of optically thin, magnetically supported, two-temperature black hole accretion disks, revealing their structure, luminosity, and temperature profiles, and explaining observed spectral state transitions.

Contribution

It introduces a comprehensive set of solutions for magnetically supported accretion disks, including the effects of magnetic fields and flux advection, extending understanding beyond traditional ADAF models.

Findings

Luminosity exceeds the maximum for ADAF, L > 0.4 α^2 L_Edd.

The low-β disk region widens with increased accretion rate.

Electron temperature decreases as luminosity increases, matching observations.

Abstract

We present global solutions of optically thin, two-temperature black hole accretion disks incorporating magnetic fields. We assume that the {\pi}{\phi}-component of the Maxwell stress is proportional to the total pressure, and prescribe the radial dependence of the magnetic flux advection rate in order to complete the set of basic equations. We obtained magnetically supported (low-{\beta}) disk solutions, whose luminosity exceeds the maximum luminosity for an advection-dominated accretion flow (ADAF), L > 0.4 {\alpha}^2 L_Edd, where L_Edd is the Eddington luminosity. The accretion flow is composed of the outer ADAF, a luminous hot accretion flow (LHAF) inside the transition layer from the outer ADAF to the low-{\beta} disk, the low-{\beta} disk, and the inner ADAF. The low-{\beta} disk region becomes wider as the mass-accretion rate increases further. In the low-{\beta} disk, the magnetic heating balances the radiative cooling, and the electron temperature decreases from ~ 10^9.5 K to ~ 10^8 K as the luminosity increases. These results are consistent with the anti-correlation between the energy cutoff in X-ray spectra (hence the electron temperature) and the luminosity when L > 0.1 L_Edd, observed in the bright/hard state during the bright hard-to-soft transitions of transient outbursts in galactic black hole candidates.