Accretion and emission processes in AGN

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 standard disk is discussed using different approximations, and it is compared to the observations. This leads to the conclusion that more complex models are required, such as the irradiated disk and the disk-corona models. The advantage of this last model is that it explains the overall UV-X spectral distribution. In the framework of these disk models, the profile, intensity, and variability properties of the X-ray iron line can be explained by reprocessing at the surface of the cold disk very close to the black hole (the "relativistic disk model"). An alternative possibility is discussed, where the UV-X continuum is produced by a quasi-spherical distribution of dense clouds surrounded by (or embedded in) a hot medium. In such a model the iron line profile could be due to Comptonization instead of relativistic effects.Comment: 55 pages, 29 figures, Lectures given at GH Advanced Lectures on the Starburst-AGN Connection, INAOE, June 2000, eds. D. Kunth, I. Aretxag

Quasars and Galactic Nuclei,a Half-Century Agitated Story

I recall how the discovery of quasars occurred more than forty years ago, and the strong debates marking out their story. It led to the discovery of Massive Black Holes, which are now known to be present in almost all galaxies, and it opened on a coherent physical model and on a new vision of galaxy evolution.Comment: 14 pages, 4 figures, talk given at the Albert Einstein Century International Conference, held in Paris, France, July 18-22, 2005, submitted to publication in AIP, Eds J.-M. Alimi and A. Fuzfa, replaced to add few references and to correct a mistak

Uncertainties of the masses of black holes and Eddington ratios in AGN

Black hole masses in Active Galactic Nuclei have been determined in 35 objects through reverberation mapping of the emission line region. I mention some uncertainties of the method, such as the ``scale factor'' relating the Virial Product to the mass, which depends on the unknown structure and dynamics of the Broad Line Region. When the black hole masses are estimated indirectly using the empirical size-luminosity relation deduced from this method, the uncertainties can be larger, especially when the relation is extrapolated to high and low masses and/or luminosities. In particular they lead to Eddington ratios of the order of unity in samples of Narrow Line Seyfert 1. As the optical-UV luminosity is provided by the accretion disk, the accretion rates can be determined and are found to be much larger than the Eddington rates. So, accretion must be performed at a super-critical rate through a slim disk, resulting in rapid growth of the black holes. The alternative is that the mass determination is wrong at this limit.Comment: 5 pages, 2 figures. To appear in the proceedings of "Black Holes: from Stars to Galaxies", IAU Symp. No. 238, V. Karas & G. Matt (eds.), Cambridge University Pres

A new type of photoionized code required for the new era of X-ray spectroscopy

With the advent of the present and future spatial X-ray missions, it becomes crucial to model correctly the line spectrum of X-ray emitting media. We have built a photoionization code, Titan, solving the transfer of a thousand lines and of the continuum with the "Accelerated Lambda Iteration" method, which is most reliable for line transfer. In all other photoionization codes the line intensities are computed with the so-called "escape probability formalism", used in its simplest approximation. In a previous paper (Dumont et al. 2003), it was shown that this approximation leads to errors which can exceed one order of magnitude in the case a Thomson thickness of the order of unity. We show here that it also happens in the case of a Thomson thickness of 0.001 to 0.1. The errors on the line fluxes and line ratios are of the order of 30% for a column density of 10^{20} cm^{-2}, and a factor five for a column density of 10^{23} cm^{-2}, in conditions appropriate for Seyfert 2 and for the Warm Absorber of Seyfert 1.Comment: 11 pages, 7 figures. accepted by A&

Galaxies et trous noirs supermassifs

International audienceA few percents of galaxies are classified as « active ». An active galaxy is a galaxy whose nucleus emits more energy than the whole galaxy in the form of electromagnetic radiation, relativistic particles, or mechanical energy. It is activated by a supermassive black hole fueled by matter falling on it, whose characteristics (Eddington luminosity, spin) are recalled. The class includes quasars and Seyfert galaxies. All massive "non active" galaxies contain a supermassive black hole, but there is not enough matter in its environment so as the nucleus becomes luminous. Different items are considered in the paper : how supermassive black holes are fueled, the accretion disc, the jets and the winds, the unified model of active galaxies, how are determined the masses of supermassive black holes, and what is the relation between the evolution of galaxies and supermassive black holes

Galaxies et trous noirs supermassifs

International audienceA few percents of galaxies are classified as « active ». An active galaxy is a galaxy whose nucleus emits more energy than the whole galaxy in the form of electromagnetic radiation, relativistic particles, or mechanical energy. It is activated by a supermassive black hole fueled by matter falling on it, whose characteristics (Eddington luminosity, spin) are recalled. The class includes quasars and Seyfert galaxies. All massive "non active" galaxies contain a supermassive black hole, but there is not enough matter in its environment so as the nucleus becomes luminous. Different items are considered in the paper : how supermassive black holes are fueled, the accretion disc, the jets and the winds, the unified model of active galaxies, how are determined the masses of supermassive black holes, and what is the relation between the evolution of galaxies and supermassive black holes