Flare-induced fountains and buried flares in AGN

Abstract

We discuss the local physical changes at the surface of an AGN accretion disk after the onset of a magnetic flare. The X-ray irradiation by a flare creates a hot spot at the disk surface where the plasma both heats up and expands in the vertical direction in order to regain the hydrostatic equilibrium. Assuming that the magnetic loop causing the flare is anchored deeply within the disk interior, we derive analytical estimates for the vertical dimension H_hot and the optical depth tau_es of the heated atmosphere as a function of the position within the spot. We perform computations for various values of the accretion rate dm/dt, the fraction f_cor of radiation dissipated within the disk corona, and the covering factor f_cover of the disk surface with flare-illuminated patches. It turns out that generally we can distinguish three characteristic radial zones within the disk showing a qualitatively different behavior of the heated material. In the innermost regions of the disk (inner zone) the expansion of the disk material is restricted by strong gravitational forces. Further out, the flare source, initially above the disk, soon becomes embedded by the expanding disk atmosphere. At these intermediate disk radii (middle zone) the material is optically thick thus greatly modifying the observed radiation by multiple Compton scattering. We show exemplary spectra models obtained from Monte Carlo simulations illustrating the trends. In the outermost regions of the disk (outer zone) the expanding material is optically thin and its influence on the observed spectra is smaller but pressure gradients in radial directions should cause the development of a fountain-like dynamical structure around the flare source. We discuss the observational consequences of our results.