RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF THE FORMATION OF HOT ACCRETION DISK CORONAE

Abstract

A new mechanism to form a magnetic pressure supported, high temperature\ncorona above the photosphere of an accretion disk is explored using three\ndimensional radiation magneto-hydrodynamic (MHD) simulations. The thermal\nproperties of the disk are calculated self-consistently by balancing radiative\ncooling through the surfaces of the disk with heating due to dissipation of\nturbulence driven by magneto-rotational instability (MRI). As has been noted in\nprevious work, we find the dissipation rate per unit mass increases\ndramatically with height above the mid-plane, in stark contrast to the\nalpha-disk model which assumes this quantity is a constant. Thus, we find that\nin simulations with a low surface density (and therefore a shallow\nphotosphere), the fraction of energy dissipated above the photosphere is\nsignificant (about 3.4% in our lowest surface density model), and this fraction\nincreases as surface density decreases. When a significant fraction of the\naccretion energy is dissipated in the optically thin photosphere, the gas\ntemperature increases substantially and a high temperature, magnetic pressure\nsupported corona is formed. The volume-averaged temperature in the disk corona\nis more than 10 times larger than at the disk mid-plane. Moreover, gas\ntemperature in the corona is strongly anti-correlated with gas density, which\nimplies the corona formed by MRI turbulence is patchy. This mechanism to form\nan accretion disk corona may help explain the observed relation between the\nspectral index and luminosity from AGNs, and the soft X-ray excess from some\nAGNs. It may also be relevant to spectral state changes in X-ray binaries.\n