arXiv subjects
Publications and source records attributed to B. Czerny.
The analysis of eight EXOSAT X-ray lightcurves of six active galactic nuclei by nonlinear prediction methods indicates that the observed short time-scale variability is truly stochastic and is not caused by deterministic chaos. This result favors X-ray emission models with mutliple centers of production of hot, possibly relativistic electrons.
We discuss the optical/UV/Xray spectra of AGN within the frame of the corona model of Witt, Czerny and Zycki (1996). In this model both the disk and the corona accrete and release energy through the viscosity. The relative strength the the disk and the corona is therefore determined by the model. Translated into the spectra, this model allows to predict the spectral index alpha_{ox}, measuring the relative strenth of the big blue bump with respect to the hard X-ray power law. Comparison of the predicted and observed distributions of alpha_{ox} allow us to conclude that the luminosity to the Eddington luminosity ratio in quasars cover typically the range 0.01 - 0.1 and it is broader in Seyferts, 0.001 - 0.3, or even higher. We identify Narrow Line Seyfert galaxies as objects having high luminosity to the Eddington luminosity ratio although, in principle, the second branch of high big blue bump objects for this ratio being below 0.001 is also predicted.
We analyse the data from the optical/IUE observational campaign of the Seyfert galaxy NGC 5548 in the context of 10 phenomenological models. On the basis of the optical/UV data as well as constraints from the X-ray observations we can favour one model of the nucleus: an accretion disc with an inner radius cutoff surrounded by a hot corona. The second acceptable model for optical/UV data is a distribution of optically thin clouds. However, X-ray constraints which were crucial in the analysis of disc type models could not have been applied; further development of this model is necessary.
We study an accretion disc with a hot continuous corona. We assume that the corona itself accretes and therefore it is powered directly by the release of the gravitational energy and cooled by radiative interaction with the disc. We consider the vertical structure of such a corona and show that the radial infall is accompanied by strong vertical outflow. Such a model has two important consequences: (i) at a given radius the corona forms only for accretion rate larger than a limiting value and the fraction of energy dissipated in the corona decreases with increasing accretion rate, and (ii) the disc spectra are significantly softer in the optical/UV band in comparison with the predictions of standard accretion discs due to the mass loss and the decrease of internal dissipation in the disc. Both trends correspond well to the mean spectra of radio quiet AGN and observed luminosity states in galactic black hole candidates.
We discuss a model for the central region of radio-quiet AGN in which coronal accretion is the source of energy for X-ray emission. We start from the Haardt & Maraschi model but supplement it with pressure balance condition, two possibilities of the angular momentum transfer and three models of the disc-corona transition. We employ the alpha-description of viscosity within the two layers. Both descriptions of the angular momentum transport lead to similar conclusions about the fraction of energy liberated in the disc although they predict strongly different properties of the disc interiors. The coupled disc/corona system has one family of solutions at low accretion rates, two families of solutions at intermediate accretion rates and no solution for high accretion rate. Of the two families of solutions, the first one has weakly dissipating coronae and the second one has coronae which dominate energetically the system. The two solutions merge at sub-Eddington accretion rate because a corona in hydrostatic equilibrium can dissipate only a limited amount of energy, which depends on the viscosity parameter.