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Accretion and Emission close to supermassive Black Holes in Quasars and AGN: Modeling the UV-X-spectrum

Abstract : With the last generation of X-ray satellites, such as XMM-Newton and Chandra, our knowledge about the X-ray properties of Active Galactic Nuclei (AGN) has greatly improved. Detailed spectroscopy and long-term observations of hundreds of kilo-seconds have provided new insights into the mechanisms of X-ray production, its modification by intervening matter, and X-ray variability.

In this thesis I present radiative transfer modeling of AGN in the Far-UV and X-ray range. The modeling considers several aspects of the observed X-ray properties: the Compton reflection/reprocessed component, X-ray variability and flares, and the effects of the warm absorber.

For the X-ray reprocessing, I investigate in detail the magnetic flare model assuming solar-like magnetic reconnections above the accretion disk. Such events should produce bright, compact sources of hard X-ray radiation creating a hot spot on the disk surface underneath. I evaluate the physical properties of the disk medium across such a spot and compute spectra of the reprocessed radiation as a function of the position in the spot and of the local viewing direction. Spots at several distances of the central black hole and for different orbital phases are considered and I vary the black hole mass, its accretion rate, and its spin-parameter. Spectra are also computed for a distant observer including a full relativistic treatment, which is based on a ray-tracing technique. I provide time-dependent simulations of single flare events taking into account the time-lags induced by the distance between the compact flare source and different positions of the hot spot. The flare modeling is conducted assuming an underlying accretion disk in hydrostatic equilibrium. The duration of the flare is supposed to be significantly smaller than the dynamical time-scale, so that the vertical density structure of the disk remains constant over the whole flare-period.

Recent observations of the Seyfert-1 galaxy MCG -6-30-15 with XMM-Newton revealed a light-curve with a bright symmetric flare over 2000 s. For this flare, Ponti et al. (2004) presented a cross-correlation analysis deriving time-lags in several energy bands. I present a simple toy model describing such time-lags by assuming that the observer detects the primary radiation and the reprocessed component as two consecutive pulses. The toy model qualitatively reproduces the observed time-lags of MCG -6-30-15 and allows an estimation of the distance between the flare source and the disk.

Using the results of the single-flare modeling, Monte-Carlo simulations for distributions of orbiting flare-spots across an accretion disk are conducted. The resulting fractional variability spectrum is constructed for different radial luminosity distributions and values of the black hole spin. These parameters are constrained for MCG -6-30-15 by applying the model to its observed fractional variability spectrum.

Finally, the modification of the X-ray radiation in regions farther away from the central object is considered by modeling the warm absorber. A grid of models is computed for a warm absorber in pressure equilibrium as recently suggested by the observation of the Seyfert-1 galaxy NGC 3783. General tendencies for the stratification of the medium and for the resulting absorption spectra are shown by varying the slope of the incident spectrum, the ionization parameter, and the column density of the warm absorber.
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Reference: Ponti, G., Cappi, M., Dadina, M., & Malaguti, G. 2004, A&A, 417, 451
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https://tel.archives-ouvertes.fr/tel-00067827
Contributor : René Goosmann <>
Submitted on : Tuesday, May 9, 2006 - 1:31:14 PM
Last modification on : Wednesday, September 23, 2020 - 4:37:38 AM
Long-term archiving on: : Saturday, April 3, 2010 - 11:20:10 PM

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René Goosmann. Accretion and Emission close to supermassive Black Holes in Quasars and AGN: Modeling the UV-X-spectrum. Astrophysics [astro-ph]. Universität Hamburg, 2006. English. ⟨tel-00067827⟩

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