On the X-ray spectra of luminous, inhomogeneous accretion flows

• Published in: Monthly notices of the Royal Astronomical Society Vol. 370; no. 4; pp. 1699 - 1712
• Main Authors: Merloni, A., Malzac, J., Fabian, A. C., Ross, R. R.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 21.08.2006; Blackwell Science; Oxford University Press; Oxford University Press (OUP): Policy P - Oxford Open Option A
• Subjects: accretion; accretion discs; Accretion disks; accretion, accretion discs; Astronomy; Astrophysics; black hole physics; Black holes; Cosmology; Earth, ocean, space; Exact sciences and technology; Sciences of the Universe; Spectrum analysis; X-ray astronomy; black hole physics; accretion, accretion discs; Black holes; Variability; Luminosity; Model study; Heat rate; Accretion disks; Thermal ionization; Reflection spectrum; Radiative properties; Active galaxy nuclei; Galaxy nuclei; Radiative transfer; Emission line; Power law; Accretion; Hard x radiation; Spectral properties; X-ray spectra; Active galaxies; Cosmology; Instability; Models; Seyfert 1 galaxies; accretion discs: black hole physics; accretion
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2006.10676.x

We discuss the expected X-ray spectral and variability properties of black hole accretion discs at high luminosity, under the hypothesis that radiation-pressure-dominated discs are subject to violent clumping instabilities and, as a result, have a highly inhomogeneous two-phase structure. After deriving the full accretion disc solutions explicitly in terms of the parameters of the model, we study their radiative properties both with a simple two-zone model, treatable analytically, and with radiative transfer simulations which account simultaneously for energy balance and Comptonization in the hot phase, together with reflection, reprocessing, ionization and thermal balance in the cold phase. We show that, if not only the density, but also the heating rate within these flows is inhomogeneous, then complex reflection-dominated spectra can be obtained for a high enough covering fraction of the cold phase. In general, large reflection components in the observed X-ray spectra should be associated with strong soft excesses, resulting from the combined emission of ionized atomic emission lines. The variability properties of such systems are such that, even when contributing to a large fraction of the hard X-ray spectrum, the reflection component is less variable than the power-law-like emission originating from the hot Comptonizing phase, in agreement with what is observed in many Narrow Line Seyfert 1 galaxies and bright Seyfert 1. Our model falls within the family of those trying to explain the complex X-ray spectra of bright AGN with ionized reflection, but presents an alternative, specific, physically motivated, geometrical set-up for the complex multiphase structure of the inner regions of near-Eddington accretion flows.