27 Ottobre 2021, ore 11:00
Title: Accretion disk instabilities in application to active galactic nuclei and changing-look AGN phenomenon
Speaker: Dr.Agnieszka Janiuk da Center for Theoretical Physics Polish Academy of Sciences, Warsaw, Poland
Abstract: Apart from regular, low-level stochastic variability, some AGN occasionally show exceptionally large changes in the luminosity, spectral shape, and/or X-ray absorption. The most notable are the changes of the spectral type when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy or vice versa. Such a phenomenon is invoked as ’Changing-Look AGN’ (CL AGN). The origin of this phenomenon is still unknown, but for most of the sources, there are strong arguments in favor of the intrinsic changes. They pose a challenge to the models of black hole accretion since the timescales of the changes are much shorter than the standard disk viscous timescales. We aim to model the CL AGN phenomenon, assuming that the underlying mechanism is the time-dependent evolution of a blackhole accretion disk unstable due to the dominant radiation pressure. We use a time-dependent, vertically integrated disk model, where we allow for the presence of the hot coronal layer above the disk. We focus on the variability timescales and amplitudes, which can be regulated by the action of large-scale magnetic fields, or in general, to the description of the disk-corona coupling. We also speculate about the presence of an inner optically thin flow (ADAF).