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Astron. Astrophys. 360, 1170-1186 (2000)


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Vertical structure of the accreting two-temperature corona and the transition to an ADAF

A. Róaska and B. Czerny

Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland (agata@camk.edu.pl)

Received 31 March 2000 / Accepted 13 June 2000

Abstract

We investigate the model of the disc/corona accretion flow around the black hole. Hot accreting advective corona is described by the two-temperature plasma in pressure equilibrium with the cold disk. Corona is powered by accretion but it also exchanges energy with the disk through the radiative interaction and conduction. The model, parameterized by the total (i.e. disk plus corona) accretion rate, [FORMULA] and the viscosity parameter, [FORMULA], uniquely determines the fraction of energy released in the corona as a function of radius and, in particular, the transition radius to the single-phase flow.

Self-consistent solutions with the mass exchange between phases display radial dependence of the parameters qualitatively different from the `static' case, without the mass exchange. Corona covers the entire disk. The character of the radial dependence of the fraction of energy dissipated in the corona is qualitatively different for low and high total accretion rate.

If the total accretion rate is low, the corona becomes stronger towards the central object, and finally the disc completely evaporates, changing the accretion pattern into the single hot advection-dominated accretion flow (ADAF). For intermediate accretion rates the reverse process - condensation - becomes important, allowing possibly for a secondary disc rebuilding in the innermost part of the system. High accretion rates always prevent the transition into ADAF, and the cold disk extends down to the marginally stable orbit.

The transition radius, [FORMULA], between the outer, two-phase flow and the inner, single-phase, optically thin flow, is equal to [FORMULA] for [FORMULA] and then contracts to the marginally stable orbit in a discontinuous way above this critical value of [FORMULA].

This model reproduces all characteristic luminosity states of accretion black hole without any additional ad hoc assumptions. In particular the mechanism of the disk evaporation leads to a new, almost horizontal branch on the accretion flow's stability curve (i.e. the dependence of accretion rate on surface density) at the critical accretion rate. This branch, together with the upper, advection dominated branch for optically thick disks, form boundaries for the time evolution of unstable, radiation pressure dominated disk. Therefore the disk at high accretion rates, corresponding to Very High State in GBH and perhaps to Narrow Line Seyfert 1, and quasar stage may oscillate between the disk dominated state and the evaporation branch state, with only a weak contribution from the cold disk emission. The position of this branch for [FORMULA] with respect to the gas pressure dominated branch is consistent with the presence of only weakly variable High State in GBH and the absence of a similar state in AGN: all the quasars vary considerably if monitored in timescales of years. We also suggest a new interpretation of the Intermediate State, consistent with the presence of the strong reflected component.

Key words: radiative transfer – accretion, accretion disks – galaxies: active – galaxies: Seyfert – X-rays: galaxies – X-rays: stars

Send offprint requests to: A. Roz_anska

© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000

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