Browsing by Author "Das, Tapas K."

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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.
Critical properties and stability of stationary solutions in multi-transonic pseudo-schwarzschild accretion
(2006-09-13) Chaudhury, Soumini; Ray, Arnab K.; Das, Tapas K.
For inviscid, rotational accretion ows, both isothermal and polytropic, a simple dynamical systems analysis of the critical points has given a very accurate mathematical scheme to understand the nature of these points, for any pseudo-potential by which the ow may be driven on to a Schwarzschild black hole. This allows for a complete classi cation of the critical points for a wide range of ow parameters, and shows that the only possible critical points for this kind of ow are saddle points and centre-type points. A restrictive upper bound on the angular momentum of critical solutions has been established. A time-dependent perturbative study reveals that the form of the perturbation equation, for both isothermal and polytropic ows, is invariant under the choice of any particular pseudo-potential. Under generically true outer boundary conditions, the inviscid ow has been shown to be stable under an adiabatic and radially propagating perturbation. The perturbation equation has also served the dual purpose of enabling an understanding of the acoustic geometry for inviscid and rotationalows.
Critical properties of spherically symmetric black hole accretion in Schwarzschild geometry
(2007-02-28) Mandal, Ipsita; Ray, Arnab K.; Das, Tapas K.
The stationary spherically symmetric accretion ﬂow in the Schwarzschild metric has been set up as an autonomous ﬁrst-order dynamical system, and it has been studied completely analytically. Of the three possible critical points in the ﬂow, the one that is physically realistic behaves like the saddle point of the standard Bondi accretion problem. One of the two remaining critical points exhibits the strange mathematical behaviour of being either a saddle point or a centre-type point, depending on the values of the ﬂow parameters. The third critical point is always unphysical and behaves like a centre-type point. The treatment has been extended to pseudo-Schwarzschild ﬂows for comparison with the general relativistic analysis.
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.
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.
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.
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.
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.
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.
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.
Simultaneous solution scheme for coupled transonic accretion-wind systems
(2000-06-15) Das, Tapas K.
We discuss a non-self-similar analytical model capable of explaining the formation of accretion-powered galactic and extra-galactic jets.
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.