2001 (IPP)
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Item Pseudo-Schwarzschild Description of Transonic Spherical Accretion onto Compact Objects(2001-03-02) Das, Tapas K.A number of ‘modified’ Newtonian potentials of various forms are available in the literature which ac- curately approximate some general relativistic effects important for studying accretion discs around a Schwarzschild black hole. Such potentials may be called ‘pseudo-Schwarzschild’ potentials because they nicely mimic the space-time around a non-rotating/slowly rotating compact object. In this paper, we examine the validity of the application of some of these potentials to study the spherically symmetric, transonic, hydrodynamic accretion onto a Schwarzschild black hole. By comparing the values of various dynamical and thermodynamic accretion parameters obtained for flows using these potentials with full general relativistic calculations, we have shown that though the potentials discussed in this paper were originally proposed to mimic the relativistic effects manifested in disc accretion, it is quite reasonable to use most of the potentials in studying various dynamical as well as thermodynamic quantities for spherical accre- tion to compromise between the ease of handling of a Newtonian description of gravity and the realistic situations described by complicated general relativistic calculations. Also we have shown that depending on the chosen regions of parameter space spanned by specific energy E and adiabatic index γ of the flow, one potential may have more importance than another and we could identify which potential is the best approximation for full general relativistic flow in Scwarzschild space-time for particular values of E and γ.Item Pseudo-Schwarzschild description of accretion-powered spherical outflow(2001-07-02) Das, Tapas K.Using two different pseudo-Schwarzschild potentials proposed by Artemova et. al, 1 we formulate and solve the equations governing spherically symmetric transonic inflow and outflow in presence of a relativistic hadronic pressure mediated steady, standing, spher- ical shock around the central compact object and then we self-consistently connect the accretion-wind solutions to calculate the mass outflow rate R ˙ m in terms of minimum number of flow parameters. Also we study the dependence of this rate on various bound- ary conditions governing the flow.Item On some transonic aspects of general relativistic spherical accretion onto schwarzschild black holes(2001-01-02) Das, Tapas K.The equations governing general relativistic, spherically symmetric, hydrodynamic accretion of polytropic fluid onto black holes are solved in Schwarzschild metric to investigate some of the transonic properties of the flow. Only stationary solutions are discussed. For such accretion, it has been shown that real physical sonic points may form even for flow with γ < 4 3 or γ > 5 3 . Behaviour of some flow variables in the close vicinity of the event horizon are studied as a function of specific energy and polytropic index of the flow.Item One parameter solution of spherically symmetric accretion in various pseudo-schwarzschild potentials(2001-03-02) Sarkar, Aveek; Das, Tapas K.In this paperwe have solved the hydrodynamic equations governing the spher- ically symmetric isothermal accretion (wind) onto (away from) compact ob- jects using various pseudo-Schwarzschild potentials.These solutions are es- sentially one parameter solutions in a sense that all relevant dynamical as well as thermodynamic quantities for such a flow could be obtained (with the as- sumption of a one-temperature fluid) if only one flow parameter (temperature of the flow T) is given. Also we have investigated the transonic behaviour of such a flow and showed that for a given T, transitions from subsonic to the supersonic branch of accretion (wind) takes place at different locations de- pending on the potentials used to study the flow and we have identified these transition zones for flows in various such potentials.Item Generalized Shock Solutions for Hydrodynamics Black Hole Accretion(2001-05-01) 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 multi-transonic 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.Item Accretion powered spherical wind in general relativity(2001-04-14) Das, Tapas K.Using full general relativistic calculations, we investigate the possibility of generation of mass outflow from spherical accretion onto non-rotating black holes. Introducing a relativistic hadronic-pressure-supported steady, standing, spherically-symmetric shock surface around a Schwarzschild black hole as the effective physical barrier that may be responsible for the generation of spherical wind, we calculate the mass outflow rate R ˙ m in terms of three accretion parameters and one outflow parameter by simultaneously solving the set of general relativistic hydrodynamic equations describing spherically symmetric, transonic, polytropic accretion and wind around a Schwarzschild black hole. Not only do we provide a sufficiently plausible estimation of R ˙ m, we also successfully study the dependence and variation of this rate on various physical parameters governing the flow. Our calculation indicates that independent of initial boundary conditions, the baryonic matter content of this shock-generated wind always correlates with post-shock flow temperature.