Accretion Disks in AGNs

We review the current state of understanding how accretion onto a black hole proceeds and what are they key elements needed to form relativistic jets. Since the theoretical progress is severely halted by the lack of deep understanding of the microphysics involved, particularly in the presence of strong magnetic fields, all proposed solutions have a status of models, and their selection has to be based directly on observational constraints.

We present a self-consistent, semi-analytical dynamical model of disk driven outflows in AGNs that are accelerated by a combination of magnetic stresses and radiation pressure. This model will make it possible to examine scenarios in which the wind is homogeneous as well as cases where it consists of dense clouds embedded in a more tenuous, magnetized medium. The various ingredients of this model will be tested through quantitative predictions from both multiwavelength spectr...

We review the results from multi-dimensional, time-dependent simulations of gas dynamics in AGN. We will focus on two types of outflows powered by radiation emitted from the AGN central engine: (i) outflows driven from the innermost part of an accretion disk and (2) outflows driven from a large-scale inflow that is likely the main supplier of material to the central engine. We discuss the relevance of both types of outflows to the so-called AGN feedback problem. However, the ...

The main characteristics of the average spectrum of radio quiet AGN in the UV and X-ray range are reviewed, and the emission mechanisms are discussed in the framework of accretion disk models, in particular the ``irradiated cold relativistic disk". It is shown that some problems arise in confronting the predictions of the model to the observations. We propose an alternative model in terms of a hot disk surrounded by a cold Compton thick medium. Finally we mention problems wit...

We study vertical structure of radiation pressure dominated disc with a hot corona. We include all the relevant processes like bound--free opacity and convection. We show that the presence of the corona modifies considerably the density and the opacity of the disc surface layers which are important from the point of view of spectrum formation. The surface of the disc with a corona is much denser and less ionized than the surface of a bare disc. Such a disc is likely to produc...

In this work, which is a continuation of Castell\'o-Mor et al. (2016), we present new X-ray and infrared (IR) data for a sample of active galactic nuclei (AGN) covering a wide range in Eddington ratio over a small luminosity range. In particular, we rigorously explore the dependence of the optical-to-X-ray spectral index $\alpha_{OX}$ and the IR-to-optical spectral index on the dimensionless accretion rate, $\dot{\mathcal{M}}=\dot{m}/\eta$ where $\dot{m}=L_{AGN}/L_{Edd}$ and ...

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 possible. After a review of standard Shakura-Sunyaev based models and their problems and uncertainties, I describe the basic principles that determine the overall spectral energy distribution produced by the flow. I then describe the phy...

Rapid, large amplitude variability at optical to X-ray wavelengths is now seen in an increasing number of Seyfert galaxies and luminous quasars. The variations imply a global change in accretion power, but are too rapid to be communicated by inflow through a standard thin accretion disc. Such discs are long known to have difficulty explaining the observed optical/UV emission from active galactic nuclei. Here we show that alternative models developed to explain these observati...

In this thesis, I study hydrodynamical models of slim accretion disks --- advective, optically thick disks which generalize the standard models of radiatively efficient thin disks to all accretion rates. I start with a general introduction to the theory of accretion onto compact objects. It is followed by a derivation of the commonly-used standard models of thin disks. In the subsequent section I introduce the equations describing slim disks, explain the numerical methods I u...

The UV-X continuum, the X-ray spectral features, and the variability in these bands provide powerful tools for studying the innermost regions of AGNs from which we gain an insight into the accretion process. In this chapter the discussion focusses on luminous AGN, i.e. Seyfert galaxies and quasars. The standard accretion disk model (a stationary geometrically thin disk) is described, and vertically averaged solutions for the radial structure are given. The emission of the sta...