Slim Accretion Disks

Instead of making the usual sort of introduction, I would like to start by presenting two figures which sum up the main ideas of the thesis and give us overall guide to the work. Figure 1 illustrates observed properties of the objects discussed here while Figure 2 is its theoretical counterpart which summarizes all of the models calculated and described in detail in this thesis. In fact, the graphs really ought to be three dimensional, with a third axis representing viscosity and I will be considering various different viscosity parameters for the models represented in Figure 2. Both of the figures show relationships between mass and accretion rate, which having in mind connotations of accretion rate, translate into mass-luminosity relations. For normal stars the mass-luminosity relation is of fundamental importance and underlies our understanding of stellar structure and evolution. What is the present state of our knowledge about the nature of galactic and extragalactic x-ray sources? Here I will concentrate on Active Galactic Nuclei and introduce a new class of theoretical models for accretion disks - slim accretion disks (Abramowicz, Czerny, Lasota and Szuszkiewicz, 1988). The simplicity of these models combined with care in treating physical phenomena in the neighbourhood of compact objects, give rise to a powerful tool for exploration within this extremely interesting field. The M(M) relation in Figure 1 has been obtained using data available in the literature. As a main source of masses and luminosities I used the paper by Padovani (1988) (a preprint of the Space Telescope Science Institute). and translated the luminosities into accretion rates using the M - L relation for slim disks. (The masses are measured in solar masses and the accretion rates in cgs units). The most interesting objects, which show evidence for the existence of accretion disks are represented in Figure 3, which shows a small part of the data in Figure 1. Among others there are Fairall 9, whose spectral properties I sturued in collaboration with Binette, Prieto and Zheng (1988 ), NGC 1566, where disk instabilities seem to explain line and continuum variability (Abramowicz, Lasota, Xu, 1986), and PG 1211+143, whose "blue bump" has been fitted by accretion disk spectra calculated by Czerny and Elvis (1987). The two straight lines in Figure 2 mark boundaries of the region in which slim accretion disk models become unstable according to the limit cycle mechanism discussed in Chapters 5 and 8 ( Abramowicz and Szuszkiewicz, 1988). This sort of inst.ability can give rise t.o quasi-periodic oscillations with timescales typically of order thousands of clays. In order to search for variability with such timescales it is crucial to increase the sample of objects for which long term regular monitoring is available. This has stimulated us (Barbieri, Cappellaro, Romano, Turatto and Szuszkiewicz, 1988) in pursuing a project of making optical observations of quasars at the Asiago Observatory. Other types of instability are discussed in Chapter 8 ( Abramowicz, Livio, Soker and Szuszkiewicz, 1988 ), and Chapter 6 contains an account of recent results for models of slim disks around supermassive black holes.