IUCAA Preprints

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    Generalized shock solutions for hydrodynamic black hole accretion
    (2002-12-15) Das, Tapas K.
    For the first time, all available pseudo-Schwarzschild potentials are exhaustively used to investigate the possibility of shock formation in hydrodynamic, invicid, black hole accretion discs. It is shown that a significant region of parameter space spanned by important accretion parameters allows shock formation for flow in all potentials used in this work. This leads to the conclusion that the standing shocks are essential ingredients in accretion discs around non-rotating black holes in general. Using a complete general relativistic framework, equations governing multitransonic black hole accretion and wind are also formulated and solved in the Schwarzschild metric. Shock solutions for accretion flow in various pseudo potentials are then compared with such general relativistic solutions to identify which potential is the best approximation of Schwarzschild space-time as far as the question of shock formation in black hole accretion discs is concerned.
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    Effect of dissipative corona on the structure and stability of cold optically thick accretion disks at high accretion rates
    (2002-03-20) Misra, Ranjeev; Taam, Ronald E.
    The vertical structure of optically thick accretion disks is investigated in the two-zone approximation. The disk is divided into an underlying disk and a corona, where the latter is defined as the upper surface layers for which the effective optical depth is unity. It is found that a significant part of the accretion flow (or dissipation rate) can take place in the corona if the scale height of the magnetic field is larger than that of the disk. The presence of such a dissipative corona leads to a modification in the topology of local disk solutions. For example, these solutions are found from local stability analysis to be both secularly and thermally stable, for accretion rates which are a factor ≈ four higher than those inferred from the stability of standard disk solutions. Thus, the applicability of optically thick disks with dissipative coronas are not as restrictive as disks without such coronas and can provide an attractive explanation for the origin of the soft spectral component observed in black hole X-ray binary systems.
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    Restrictions on the Physical Prescription for the Viscosity in Advection-Dominated Accretion Disks
    (2011-07-06) Becker, Peter A.; Subramanian, Prasad
    It has recently been demonstrated that the Shakura-Sunyaev prescription for the kinematic viscosity in an advection-dominated accretion disk yields physically reasonable solutions for the structure of the inflow close to the event horizon. In particular, no violations of relativistic causality occur at the horizon. This is somewhat surprising considering the diffusive nature of the angular momentum transport in the Shakura-Sunyaev scenario, and it is therefore natural to ask whether one can also obtain acceptable solutions for the disk structure based on the various alternative models for the viscosity that have been proposed, includ- ing the “deterministic” forms. In this paper we perform a rigorous asymptotic analysis of the structure of an advection-dominated accretion disk close to the event horizon of a nonrotating black hole based on three of the alternative pre- scriptions for the viscosity that have been suggested in the literature. We constrain the physical disk model by stipulating that the stress must van- ish at the horizon, which is the fundamental inner boundary condition imposed by general relativity. Surprisingly, we find that none of the three alternative viscosity prescriptions yield physically acceptable disk structures close to the horizon when the zero-torque condition is applied, whether the flow is in vertical hydrostatic equilibrium or free-fall. Hence we conclude that the original Shakura- Sunyaev prescription is the only one proposed so far that is physically consistent close to the event horizon. We argue that, somewhat ironically, it is in fact the diffusive nature of the Shakura-Sunyaev form that is the reason for its success in this application. Our focus here is on advection-dominated accretion disks, but we expect that our results will also apply to generalized disks provided that losses of matter and energy become negligible as the gas approaches the event horizon.