Physics of accretion disks with magnetic flares

Abstract

Rapid progress in multi-wavelength observations of Seyfert Galaxies in recent years is providing evidence that X-ray emission in these objects may be produced by magnetic flares occurring above a cold accretion disk. Here we attempt to develop a physically consistent model of accretion disks producing radiation via magnetic flares as well as the optically thick intrinsic disk emission, and apply this model to observations of Active Galactic Nuclei (AGN) and Galactic Black Hole Candidates (GBHCs). The following issues are considered: (1) the pressure equilibrium in the flare region, (2) the reflection and reprocessing of the X-radiation from flares in the underlying disk, (3) the spectra of GBHCs in the context of the model, (4) and the generation of the flares by the disk--the energy budget of the corona. Our results show that: (1) The temperature of the disk atmosphere near active magnetic flares in AGN is in the range 1 - 3 x 10⁵ Kelvin, and that the material is relatively non-ionized. This temperature is in a good agreement with the observed rollover energy in the Big Blue Bump (BBB) of Seyfert 1 Galaxies. We thus suggest that the BBB is simply the X-rays from magnetic flares reprocessed into the X-ray skin of the accretion disk. (2) We suggest an explanation for the recently discovered X-ray Baldwin effect and the controversy over the existence of BBBs in quasars more luminous than typical Seyferts. (3) Due to an ionization instability and much higher X-ray incident flux, we found that the X-ray skin in GBHCs is nearly completely ionized. Using an approximate model to describe this effect, we calculated the reflected/reprocessed spectrum and the resulting corona spectrum simultaneously. We found that the spectrum of GBHCs in their hard state may be explained with this model, with basically the same parameters for magnetic flares as in the AGN case. (4) The magnetic energy transport is shown to be large enough to account for the observed amount of X-rays from Seyferts and GBHCs. We predict that X-ray spectra are hard for accretion rates below the gas-to-radiation transition, and that they are softer above this transition. (5) We collected our results into a diagram that shows how the observational appearance of accreting black holes changes with the accretion rate and the mass of the hole, and compared it with observations of AGN and GBHCs. Our conclusion is that the agreement between theory and observations is very encouraging and we suggest that the physics of magnetic flares is the physics that should be added to the standard accretion disk theory in order to produce a more realistic description of accretion flows with large angular momentum.