Abstract
We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity parameter $\alpha$. For $\dot M<\dot M_{crit}$ we show that there are three equilibrium branches of solutions. One of the branches corresponds to a cool optically thick flow which is the well-known thin disk solution of Shakura \& Sunyaev (1973). Another branch corresponds to a hot optically thin flow, discovered originally by Shapiro, Lightman \& Eardley (1976, SLE). This solution is thermally unstable. The third branch corresponds to our new advection-dominated solution. This solution is hotter and more optically thin than the SLE solution, but is viscously and thermally stable. It is related to the ion torus model of Rees et al. (1982) and may potentially explain the hard X-ray and $\gamma$-ray emission from X-ray binaries and active galactic nuclei.
Highlights
One of the cornerstones of the theory of accretion disks is the model of thin disks developed by Shakura & Sunyaev (1973), Novikov & Thorne (1973), and Lynden-Bell & Pringle (1974)
As we have discussed in Papers 1 and 2, advection-dominated flows can in principle occur either in systems with very high mass accretion rates (Begelman 1978), where the photon diffusion time scale is very long, or in systems with very low mass accretion rates (Rees et al 1982), where the local optically thin cooling time scale becomes very long
We model the gas as a twotemperature plasma where the ions and electrons are allowed to have different temperatures, determined by individual thermal balance equations for each species
Summary
One of the cornerstones of the theory of accretion disks is the model of thin disks developed by Shakura & Sunyaev (1973), Novikov & Thorne (1973), and Lynden-Bell & Pringle (1974) (see Frank, King & Raine 1992 for a review). The thermal instability arises because the accreting gas is optically thin so that the cooling efficiency via bremsstrahlung decreases with decreasing density (Pringle, Rees & Pacholczyk 1973, Piran 1978) Because of this feature, if an equilibrium SLE flow is perturbed to a slightly higher temperature, its density goes down and the rate of cooling decreases. Following up on the Narayan & Popham (1993) work, the present authors investigated in Papers 1 and 2 the general properties of advection-dominated flows In these papers we derived self-similar solutions, both of the slim disk equations (Paper 1) and of a more general non-height-integrated set of equations (Paper 2). We use a Comptonization formula which automatically works both in the unsaturated and saturated limits
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