IUCAA Preprints

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    Radiating Kerr-Newman black hole in f(R) gravity
    (IUCAA, 2015-02) Ghosh, S. G.; Maharaj, S. D.; Papnoi, Uma
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    Spinning Higher Dimensional Einstein-Yang-Mills black holes
    (IUCAA, 2015-02) Ghosh, S. G.; Papnoi, Uma
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    Negative Energy Condition and Black Holes on the Brane
    (2000-02-24) Dadhich, Naresh
    We propose that for non-localizable energy distribution the rele- vant energy condition is determined by the gravitational field energy which is negative for positive non-gravitational energy. That is neg- ativity of the non-localized energy is the ”positive” energy condition. This would have direct application and relevance for a black hole on the brane which would be sitting in a trace free stresses induced by the Weyl curvature of the bulk.
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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.
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    Acoustic perturbations on steady spherical accretion in Schwarzschild geometry
    (2007-06-10) Naskar, Tapan; Ray, Arnab K.
    The stationary background flow in the spherically symmetric infall of a compressible fluid, coupled to the space-time defined by the static chwarzschild metric, has been subjected to linearized acoustic perturbations. The perturbative procedure is based on the continuity condition and it shows that the coupling of the flow with the geometry of space-time brings about greater stability for the flow, to the extent that the amplitude of the perturbation, treated as a standing wave, decays in time, as opposed to the amplitude remaining constant in the Newtonian limit. In ualitative terms this situation simulates the effect of a dissipative mechanism in the classical Bondi accretion flow, defined in the Newtonian construct of space and time. As a result of this approach it becomes impossible to define an acoustic metric for a conserved spherically symmetric flow, described within the framework of Schwarzschild geometry. In keeping with this view, the perturbation, considered eparately as a high-frequency travelling wave, also has its amplitude reduced.
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    5D radiating black holes in einstein-yang-mills-gauss-bonnet gravity
    (2011-08-25) Ghosh, S. G.
    We derive nonstatic spherically symmetric solutions of a null uid, in ve dimension (5D), to Einstein-Yang-Mills (EYM) equations with the coupling of Gauss-Bonnet (GB) combination of quadratic curvature terms, namely, 5D-EYMGB radiating black hole solution. It is shown that, in the limit, we can recover known radiating black hole solutions. The spherically symmetric known 5D static black hole solutions are also retrieved. The e ect of the GB term and Yang-Mills (YM) gauge charge on the structure and location of horizons, of the 5D radiating black hole, is also discussed.