Institute for Advanced Study Astrophysics Seminar

The alpha disk model is widely used to describe the accretion disk structures around supermassive black holes, even though it cannot explain many observed properties of AGNs. Various alternative models have been proposed including the ones where magnetic pressure dominates over the thermal pressure. In this talk, I will show that radiation spectrum is the key to distinguish these models.  I will describe our series of global 3D radiation MHD simulations of  AGN accretion disks, focusing on the radial range where UV photons are expected to be produced. The disks reach accretion rates ranging from 0.03 to 4 times the Eddington value. The disks become radiation pressure or magnetic pressure dominated depending on the relative timescales of radiative cooling and gas inflow. Magnetic pressure supported disks can form either with or without net poloidal magnetic fields as long as the inflowing gas can cool quickly enough, which can typically happen when the accretion rate is low.  We calculate the emerging spectra from these disks using multi-group radiation transport with realistic opacities and find that they typically peak around 10 eV.  At accretion rates close to or above the Eddington limit, a power-law component can appear for photon energies between 10 eV and 1 keV with a spectral slope varying between L_\nu\propto\nu^{-1} and \nu^{-2}, comparable to what is observed in radio quiet quasars.  These high energy photons are produced in an optically thick region~ 30^{\circ}-45^{\circ} from the disk midplane  by compressible bulk Comptonization within the converging accretion flow. Strongly magnetized disks that have a very small surface density will produce a spectrum that is very different from what is observed.