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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 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 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 Discovery of giant 'radio arcs' in cluster Abell 3376 : evidence for shock accleration in a violent cluster merger?(2002-10-24) Bagchi, JoydeepNew multi-wavelength (radio, optical & X-rays) observational evidences are presented which show that the nearby (z = 0.046), rich cluster of galaxies Abell 3376 is experiencing a major event of binary subcluster merger. The key evidence is the discovery of a pair of large, optically unidentified diffuse radio sources (‘arcs’), symmetrically located about 2.6 h−1 50 Mpc apart at the opposite ends of the hot intra-cluster gas mapped by ROSAT in X-rays. It is argued that the gas-dynamical shock-waves, which occur naturally during cluster formation, are accelerating charged particles (cosmic rays) to relativis- tic energies, leading to synchrotron emission from the megaparsec scale radio arcs. If this is so, cluster Abell 3376 would also be a potential source capa- ble of accelerating cosmic ray particles upto ultra-high energies (UHECR) of Emax ∼ 1018−19 eV. Thus this cluster is an excellent test-bed for understand- ing the physics of merger shocks and origin of enigmatic UHECR particles in structure formation process. Hence, Abell 3376 provides unique oppurtu- nities for further multi-wavelength observations with ground and space-borne observatoriesItem 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 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