Recent papers on astro-ph
The Origin of Jets
Accretion Disks THEORY
Theory of magnetized advection dominated accretion flows
ADVECTIVE ACCRETION DISKS
AND RELATED PROBLEMS INCLUDING MAGNETIC FIELDS
Accretion disk theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disk. One of the most important new invention of this theory was the phenomenological treatment of the turbulent viscosity, known the "alpha" prescription, where the (r f) component of the stress tensor was approximated by (a p) with a unknown constant a. This prescription played the role in the accretion disk theory as well important as the mixing-length theory of convection for stellar evolution. Sources of turbulence in the accretion disk are discussed, including hydrodynamical turbulence, convection and magnetic field role. In parallel to the optically thick geometrically thin accretion disk models, a new branch of the optically thin accretion disk models was discovered, with a larger thickness for the same total luminosity. The choice between these solutions should be done of the base of a stability analysis. The ideas underlying the necessity to include advection into the accretion disk theory are presented and first models with advection are reviewed. The present status of the solution for a low-luminous optically thin accretion disk model with advection is discussed and the limits for an advection dominated accretion flows (ADAF) imposed by the presence of magnetic field are analysed. Related problems of mass ejection from accretion disks and jet formation are discussed.
II. Magnetic Field Limitations on ADAF
Observational evidence for a black hole in the center of our Galaxy and in the nuclei of other galaxies (Cherepashchuk 1996; Haswell 1999) make it necessary to revise or generalize theoretical models of accretion flows. Improvements of the models include account of advective terms and account of the influence of equipartition magnetic fields. Conclusions based on ADAF solutions for optically thin accretion flows at low mass accretion rates are at present open to question. This is because the ADAF solutions neglect the unavoidable magnetic field annihilation, which may give significant electron heating. In contrast with the ADAF solutions, we argue that the radiative efficiency is >25% of the standard value (for an optically thick, geometrically thin disk). For equipartion between magnetic and kinetic energies, we argue that half of the dissipated energy of the accretion flow results from the destruction of the magnetic field. Further, we give reasons that suggest that at least half of this energy goes into accelerating electrons in reconnection events analogous to those in the corona of the Sun. It is possible that a full treatment of the ion-electron energy exhange due to the plasma turbulence further increases the radiative efficiency to its standard value (see also Fabian & Rees 1995).
Some observational data that were interpreted as evidence for the existence of the ADAF regime have disappeared after additional accumulation of data. The most interesting example of this sort is connected with the claim of "proof" of the existence of event horizons of black holes due to manifestation of the ADAF regime of accretion (Narayan, Garcia, & McClintock 1997). Analysis of a more complete set of observational data (Chen et al. 1997) shows that the statistical effect claimed as an evidence for ADAF disappears. This example shows the danger of "proving" a theoretical model with preliminary observational data. It is even more dangerous when the model is physically not fully consistent, because then even a reliable set of the observational data cannot serve as a proof of the model.
Thus, there are fundamental reasons for questioning the application ADAF models to underluminous AGNs, where the observed energy flux is much smaller than expected for standard accretion disk models. Two possible explanations may be suggested. One is based on more accurate estimations of the accretion mass flow into the black hole, which could be overestimated. Another possibility is based on existence of jets and/or outflows that expel most of the matter supplied at large distances Maccr Bisnovatyi-Kogan 1999. The accretion to the black hole may be much smaller than Maccr with the result that the accretion luminosity is much smaller without the radiative efficiency being small. (The model of Blandford & Begelman 1999 takes into account outflows, but requires the same assumption as earlier ADAF models that there is no heating of electrons. BKL and the present work argue against this assumption.) Many compact astrophysical objects are observed to have jets or outflows, including active galaxies and quasars, old compact stars in binaries, and young stellar objects. The formation of jets and outflows is very probable under conditions where an ordered magnetic field threads a thin disk (see reviews by Bisnovatyi-Kogan 1993; Lovelace, Ustyugova, & Koldoba 1999). To extend this interpretation, we suggest that underluminous AGNs may lose the main part of their energy to the formation of jets or outflows. This suggests a search for a correlation between existence of jets or outflows and underluminous galactic nuclei.
III. INFLUENCE OF OHMIC HEATING ON ADVECTION-DOMINATED ACCRETION FLOWS
This work considers magnetized advection-dominated accretion flows where the magnetic field is in equipartition with the turbulent motions of the flow (Shvartsman 1971). The magnetic energy density of the flow must be dissipated by ohmic heating with a rate comparable to that of the viscous dissipation (Bisnovatyi-Kogan & Ruzmaikin 1974). We argue that the ohmic and viscous dissipation must occur as a result of plasma instabilities. Further, we argue that the instabilities are likely to be current driven in response to the electric field (associated with the turbulent motion), which has a significant component parallel to the magnetic field. These instabilities are likely to heat mainly the electrons. We have analyzed a model for the radial variation of the electron and ion temperatures assuming that a constant fraction g of the viscous plus ohmic heating goes into heating the electrons and that a fraction (1 - g) goes into heating the ions. In contrast with Narayan & Yi (1995), we do not assume that a constant fraction f of the dissipated energy is advected inward by the flow. The electrons cool by bremsstrahlung and cyclotron radiation and exchange energy with the ions by Coulomb collisions. At large accretion rates M , Coulomb collisions act to give Ti ~ Te, high radiative efficiency, and geometrically thin, optically thick disk accretion. For small accretion rates, where advection-dominated accretion flows are suggested to occur, and only Coulomb energy exchange between ions and electrons, a regime of optically thin accretion flows with a large difference between ion and electron temperatures (Te << Ti) exists (Shapiro, Lightman, & Eardley 1976). Here, we emphasize that the accretion flow properties depend critically on the ohmic heating of the electrons. For small accretion rates where the electron temperature is much less than the ion temperature, we show that the ohmic heating of the electrons gives a radiative efficiency that is reduced by a factor of g from that for a thin disk. Thus, the tiny radiative efficiencies (<10-3) found by Narayan & Yi (1995) correspond to tiny values of g, which are unlikely for the reasons discussed in § 2.
Plasma instabilities due to electron-ion streaming (for electron drift velocity larger than the ion thermal speed) may greatly enhance the energy exchange between ions and electrons. In this case the two-temperature regime disappears, the ion and electron temperatures collapse to small values, T i,e << 1, and the disk is geometrically thin, that is, advection-dominated accretion flows do not occur (Fabian & Rees 1995).