Evolution of bimodal accretion flows

Abstract

We consider time-dependent models for rotating accretion flows onto black holes, where a transition takes place from an outer cooling-dominated disc to a radiatively inefficient flow in the inner region. In order to allow for a transition of this type we solve the energy equation, both, for the gas and for the radiation field, including a radiative cooling flux and a turbulent convective heat flux directed along the negative entropy gradient. The transition region is found to be highly variable, and a corresponding variation expected for the associated total luminosity. In particular, rapid oscillations of the transition radius are present on a timescale comparable with the local Keplerian rotation time. These oscillations are accompanied by a quasi-periodic modulation of the mass flux at the outer edge of the advection-dominated accretion flow (ADAF). We speculate about the relevance for the high-frequency QPO phenomenon.Comment: 6 pages, 3 figures. Revised version, added system of equations. Accepted for publication in MNRA

Short-term optical variability of high-redshift quasi-stellar objects

“…Another possible explanation of the variations is based on the instabilities of the flow itself, rather than on reprocessing. If we assume that the inner part of the flow operates through an optically thin advective mode [the advection‐dominated accretion flow (ADAF); Narayan, Mahadevan & Quataert 1998], the border between the ADAF and the outer thin disc may be a good candidate for the region where these instabilities occur (Gracia et al 2003; Krishan, Ramadurai & Wiita 2003). Again, FUV variations will be effectively generated if this transition happens to occur close to r UV (see above).…”

Disk Corona Interaction: Mechanism for the Disk Truncation and Spectrum Change in Low-Luminosity Active Galactic Nuclei

“…Early work by Wandel & Liang (1991) demonstrated the existence of multiple solutions in a hybrid disk model, which included a hot inner disk that could, in principle, be connected to a cool outer disk for a viscous stress proportional to gas pressure (see also Hubeny 1990). However, the preference for this solution over a purely optically thick gas pressure solution was not addressed Narayan & Yi (1995) Disk evaporation Meyer et al (2000aMeyer et al ( , 2000b, Ròżaǹska & Czerny (2000a, Spruit & Deufel (2002), and Dullemond & Spruit (2005) Radial conduction Honma (1996), Manmoto & Kato (2000), Gracia et al (2003) Thermal instability Takeuchi & Mineshige (1998), Gu & Lu (2000), Lu et al (2004) and was only hypothesized by Narayan & Yi (1995) in the "strong ADAF principle" (if an ADAF can form, it will). Mechanisms to facilitate the truncation of a thin disk at low accretion rates have been investigated by a number of groups including Honma (1996), Manmoto & Kato (2000), Lu et al (2004), Meyer et al (2000aMeyer et al ( , 2000b, Ròżaǹska & Czerny (2000a, Spruit & Deufel (2002), and Dullemond & Spruit (2005).…”

Magnetohydrodynamic instability in differentially rotating compressible flow

“…The behaviour of hydromagnetic instability is investigated in the region described by Ω 2 e < 0. Though Ω 2 e < 0 is generally not satisfied in the accretion disc, but there are possibilities that Ω 2 e < 0 is locally satisfied (Honma 1996; Kato & Manmoto 2000; Manmoto & Kato 2000; Gracia et al 2003) in the accretion discs. The present work will be applicable in those regions (Kato, private communication).…”