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The geometry of the accretion flow around stellar mass and supermassive black holes depends on the accretion rate. Broad iron emission lines originating from the irradiation of cool matter can indicate that there is an inner disk below a hot coronal flow.These emission lines have been detected in X-ray binaries. Observations with the Chandra X-ray Observatory, XMM Newton and Suzaku have confirmed the presence of these emission lines also in a large fraction of Seyfert-1 active galactic nuclei (AGN). We investigate the accretion flow geometry for which broad iron emission lines can arise in hard and soft spectral state. We study an ADAF-type coronal flow, where the ions are viscously heated and electrons receive their heat only by collisions from the ions and are Compton cooled by photons from an underlying cool disk. For a strong mass flow in the disk and the resulting strong Compton cooling only a very weak coronal flow is possible. This limitation allows the formation of ADAF-type coronae above weak inner disks in the hard state, but almost rules them out in the soft state. The observed hard X-ray luminosity in the soft state, of up to 10% or more of the total flux, indicates that there is a heating process that directly accelerates the electrons. This might point to the action of magnetic flares of disk magnetic fields reaching into the corona. Such flares have also been proposed by observations of the spectra of X-ray black hole binaries without a thermal cut-off around 200 keV.
We use global three dimensional radiation magneto-hydrodynamical simulations to study accretion disks onto a $5times 10^8M_{odot}$ black hole with accretion rates varying from $sim 250L_{Edd}/c^2$ to $1500 L_{Edd}/c^2$. We form the disks with torus c
In most accreting black-hole systems the copious X-rays commonly observed from the inner-most regions are accompanied by a reflection spectrum. The latter is the signature of energetic photons reprocessed by the optically thick material of an accreti
The analysis of the thermal spectrum of geometrically thin and optically thick accretion disks of black holes, the so-called continuum-fitting method, is one of the leading techniques for measuring black hole spins. Current models normally approximat
We study the structure of accretion disks around supermassive black holes in the radial range $30sim 100$ gravitational radii, using a three dimensional radiation magneto-hydrodynamic simulation. For typical conditions in this region of Active Galact
These lectures provide an overview of the theory of accretion disks with application to bright sources containing black holes. I focus on the fundamental physics of these flows, stressing modern developments and outstanding questions wherever possibl