Why Canonical Disks Cannot Produce Advection-dominated Flows

Molteni, D.; Gerardi, G.; Valenza, M.

doi:10.1086/319850

Cited by 14 publications

(17 citation statements)

References 21 publications

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“…This is similar to the procedure followed by Narayan et al (1997), except that they included an additional differential equation for a(r), based on the entropy equation (14). However, this extra differential equation is not necessary because the energy flow rate _ E E is conserved when radiative losses are neglected, as assumed in the ADAF scenario (Molteni, Gerardi, & Valenza 2001). This fact allows us to solve for a as an algebraic function of v, ', and r using equation (44), which yields…”

Section: Dynamical Equation and Critical Conditionsmentioning

confidence: 99%

Inner Boundary Conditions for Advection‐dominated Accretion onto Black Holes

Becker

2003

ApJ

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The structure of the inner region of an advection-dominated accretion disk around a nonrotating black hole is explored by applying asymptotic analysis in the region just outside the event horizon. We assume that the viscous transport is described by the standard Shakura-Sunyaev prescription throughout the disk, including the inner region close to the horizon. One of our goals is to explore the self-consistency of this assumption by analyzing the causality of the viscous transport near the black hole. The effects of general relativity are incorporated in an approximate manner by utilizing a pseudo-Newtonian gravitational potential. Analysis of the conservation equations yields unique asymptotic forms for the behaviors of the radial inflow velocity, density, sound speed, and angular velocity. The specific behaviors are determined by three quantities, namely, the accreted specific energy, the accreted specific angular momentum, and the accreted specific entropy. The additional requirement of passage through a sonic point further constrains the problem, leaving only two free parameters. Our detailed results confirm that the Shakura-Sunyaev viscosity yields a well-behaved flow structure in the inner region that satisfies the causality constraint. We also show that the velocity distribution predicted by our pseudo-Newtonian model agrees with general relativity in the vicinity of the horizon. The asymptotic expressions we derive therefore yield useful physical insight into the structure of advection-dominated disks, and they also provide convenient boundary conditions for the development of global models via numerical integration of the conservation equations. Although we focus here on advection-dominated flows, the results we obtain are also applicable to disks that lose matter and energy, provided that the loss rates become negligible close to the event horizon.

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Section: Dynamical Equation and Critical Conditionsmentioning

confidence: 99%

Inner Boundary Conditions for Advection‐dominated Accretion onto Black Holes

Becker

2003

ApJ

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“…These shocks produce around the central object a bulge of hot gas comptonizing the low‐energy thermal emission from a cold Keplerian disc. We remark that shocks produce the hot corona in a quite natural way, while the corona outcome or the ADAF solutions originating from Keplerian discs have been shown by Molteni, Gerardi & Valenza (2001b) to be very unlikely.…”

Section: Discussionmentioning

confidence: 82%

Steady shocks around black holes produced by sub-Keplerian flows with negative energy

Molteni

Gerardi

Teresi

2005

Monthly Notices of the Royal Astronomical Society

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We discuss a special case of formation of axisymmetric shocks in the accretion flow of ideal gas on to a Schwarzschild black hole: when the total energy of the flow is negative. The result of our analysis enlarges the parameter space for which these steady shocks are exhibited in the accretion of gas rotating around relativistic stellar objects. Since Keplerian discs have negative total energy, we guess that, in this energy range, the production of the shock phenomenon might be easier than in the case of positive energy. So our outcome reinforces the view that sub‐Keplerian flows of matter may significantly affect the physics of the high energy radiation emission from black hole candidates. We give a simple procedure to obtain analytically the position of the shocks. The comparison of the analytical results with the data of one‐dimensional (1D) and two‐dimensional (2D) axisymmetric numerical simulations confirms that the shocks form and are stable.

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“…These shocks produce around the central object a bulge of hot gas comptonizing the low-energy thermal emission from a cold Keplerian disc. We remark that shocks produce the hot corona in a quite natural way, while the corona outcome or the ADAF solutions originating from Keplerian discs have been shown by Molteni, Gerardi & Valenza (2001b) to be very unlikely. We stress that these flows are essentially axisymmetric.…”

Section: O N C L U S I O N Smentioning

confidence: 82%