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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 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 Stability of accretion disk around rotating black holes: a pseudo-general-relativistic fluid dynamical study(2012-03-13) Mukhopadhyay, B.We discuss the solution of accretion disk when the black hole is chosen to be rotating. We study, how the fluid properties get affected for different rotation parameters of the black hole. We know that no cosmic object is static in Universe. Here the effect of the rotation of the black hole to the space-time is considered following an earlier work of the author, where the pseudo-Newtonian potential was prescribed for the Kerr geometry. We show that, with the inclusion of rotation of the black hole, the valid disk parameter region dramatically changes and disk becomes unstable. Also we discuss about the possibility of shock in accretion disk around rotating black holes. When the black hole is chosen to be rotating, the sonic locations of the accretion disk get shifted or disappear, making the disk unstable. To bring it in the stable situation, the angular momentum of the accreting matter has to be reduced/enhanced (for co/counter-rotating disk) by means of some physical processItem Thermally driven outflows from pair-plasma pressure-mediated shock surfaces around schwarzschild black holes(2000-01-23) Das, Tapas K.Introducing a spherical, steady, self-supported pair-plasma pressure-mediated shock surface around a Schwarzschild black hole as the effective physical atmosphere that may be responsible for the generation of astrophysical mass outflows from relativistic quasi- spherical accretion, we calculate the mass outflow rate Rm Ç by simultaneously solving the set of equations governing transonic polytropic accretion and isothermal winds. Rm Ç is computed in terms of only three inflow parameters, which, we believe, has been done for the first time in our work. We then study the dependence of Rm Ç on various inflow as well as shock parameters, and establish the fact that the outflow rate is essentially controlled by the post- shock proton temperature.Item On the formation of accretion-powered galactic and extra-galactic jets(2000-09-20) Das, Tapas K.Though widely observed to be emanating from a variety of astrophysical sources, the underlying physical mechanism behind the formation of galactic and extragalactic outflows is still enshrouded in a veil of mystery. In addition, it has not been possible to calculate accurately the amount of matter expelled in these events. In this article we present a non-self-similar analytical model, which, for the first time, we believe is able to explain the outflow formation phenomenon as well as compute the mass outflow rate by simultaneously solving the equations governing the exact transonic accretion and outflow. Our model predicts the dependence of this rate on various flow parameters as well as indicates the exact location from where the outflows are launched.Item 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.Item 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.Item Unification to the Pseudo-General-Relativistic Analysis of Accretion Disks around rotating Black Holes and Neutron Stars(2011-07-05) Mukhopadhyay, B.I analyse the relativistic accretion phenomena around rotating black holes and neutron stars and show both the kinds of disk can be treated in an unified manner. The corresponding accretion disks are described by pseudo-Newtonian approach. For this purpose, number of pseudo-Newtonian potentials are in literature, applicable to describe the relativistic properties of accretion disk. While, Kerr metric is used to describe the pseudo-Newtonian potential for accretion disk around black hole, the Hartle-Thorne metric is considered to describe disk around neutron star as the metric can describe continuously the space-time, inside the star as well as out-side of it. Two other potentials were proposed to describe the temporal effects of the accretion disk. All the potentials reproduce the marginally stable and bound orbits approximately or exactly as that of general relativity. These also reproduce the specific mechanical energy approximately. Using these potentials, I study the global parameter space of the accretion disk around black holes and neutron stars. I study, how the fluid properties get affected for different angular momentum of the compact object. I show that, for different angular momenta of the compact object, the valid disk parameter region dramatically changes and disk may become unstable in certain situations. Also I discuss about the possibility of shock in accretion disk around rotating black holes and neutron stars. When the angular momentum of compact object is chosen to be varied, the sonic locations of the accretion disk get shifted or disappear, making the disk unstable. To bring it in the stable situation, the angular momentum of the accreting matter has to be reduced/enhanced (for co/counter-rotating disk) by means of some physical process. I also study, how the fluid properties get changed with different rotations of the black holes, neutron stars and other gravitating central stars. Moreover, I show the effect of viscosity to the fluid properties of the disk. Thus, I find out the unified physical parameter regime, for which the stable accretion disk can be formed. Subsequently, a theoretical prediction of kHz QPO is given, for a fast rotating compact object as 4U 1636-53.Item Origin and interpretation of kilohertz QPOs from strange stars in X-ray Binary system: Theoretical hydrodynamical description(2011-07-05) Mukhopadhyay, B.; Ray, Subharthi; Dey, Jishnu; et al.We model and interpret the Kilohertz QPOs from the hydrodynamical description of accretion disk around a rapidly rotating compact strange star. The higher QPO frequency is described by the viscous effects of accretion disk leading to shocks, while the lower one is taken to be the Keplerian motion of the accreting matter. Comparing our results with the observations for two of the fastest rotating compact stellar candidates namely, 4U 1636−53 and KS 1731−260, we find that they match to a very good approximation, thus interpreting them as strange stars.Item Global solution of viscous accretion disk around rotating compact objects : A pseudo-general-relativistic study(2011-07-05) Mukhopadhyay, B.; Ghosh, ShubhrangshuWe study the solution of viscous accretion disks around rotating com-pact/central object having hard surface i.e. neutron star, strange star and any other highly gravitating objects. We choose pseudo-Newtonian approach to describe the relativistic accretion disk. For this purpose, a new pseudo- Newtonian potential is established which is applicable to describe the rela- tivistic properties of star and its disk. As we know, the Hartle-Thorne met- ric can describe geometry of star as well as the space-time out-side of it, we use this metric to establish our potential. Our potential reproduces the marginally stable orbit exactly as that of general relativity. It also reproduces the marginally bound orbit and specific mechanical energy at the marginally stable orbit with at most 4% and 10% error respectively. Using this poten- tial we study the global parameter space of the accretion disk. Thus, we find out the physical parameter regime, for which the stable accretion disk can be formed around gravitating object with hard surface. We also study, how the fluid properties get changed with different rotations of the central star. We show that with the change of rotation to the central object, the valid disk parameter region dramatically changes.We also show the effect of viscosity to the fluid properties of the disk. Subsequently, we give a theoretical prediction