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Tapas Kumar Das

Publications and source records attributed to Tapas Kumar Das.

32 records · Page 2Linked to original sources
arXiv2003

Behaviour of matter close to the event horizon

Investigation of the behaviour of accreting matter close to the black hole event horizon is of fundamental importance in relativistic and high energy astrophysics because they provide the key features of the diagnostic spectra of the stellar mass and super-massive black holes. In this paper, we examine the terminal behaviour of general relativistic matter in multi-transonic, advective black hole accretion discs. We compute, for the first time we believe, the values of various dynamical and thermodynamic multi-transonic flow variables {\it extremely close} ($\le$ 0.01 $r_g$) to the event horizon and study the dependence of these variables on fundamental accretion parameters. Our calculation is useful for a better understanding of Hawking radiation from acoustic black holes.

astro-ph↗
arXiv2003

QPO frequencies and mass-outflow rates in black hole powered galactic microquasars

For all available pseudo-Schwarzschild potentials, we provide a non-self-similar model of coupled accretion-outflow system in connection to the Quasi Periodic Oscillation (QPO) of the black hole powered galactic microquasars and the emergence of barionic jets out of these objects. We use the vertically integrated 1.5 dimensional model to describe the disc structure where the equations of motion are written on the equatorial plane of the central accretor, assuming the flow to be in hydrostatic equilibrium in the transverse direction. First we formulate and solve the equations governing axisymmetrically rotating, advective, multi-transonic black hole accretion which may contain the Rankine Hugoniot Shock Waves (RHSW), and then we calculate the associated QPO frequencies $ν_{QPO}$ in terms of relevant accretion parameters. We then argue that the post-shock region for such flows may serve as an efficient source of outflow generation; and we calculate, for {\it same} set of accretion parameters used to calculate the $ν_{QPO}$s, what fraction of the accreting material, denoted by $R_{\dot m}$, is being blown as shock generated outflow. In this way we theoretically study the relation between $ν_{QPO}$s and $R_{\dot m}$s for galactic microquasars and compare our theoretically obtained result with observational data.

astro-ph↗
arXiv2003

Multi-transonic Black Hole Accretion Discs with Isothermal Standing Shocks

In this work we would like to address the issue of shock formation in black hole accretion discs. We provide a {\it generalized two parameter solution scheme} for multi-transonic, accretion and wind around Schwarzschild black holes, by mainly concentrating on accretion solutions which may contain steady, standing isothermal shocks. Such shocks conserve flow temperature by the dissipation of energy at the shock location.Unlike previous works in this field, our calculation is {\it not} restricted to any particular kind of post-Newtonian gravitational potentials, rather we use {\it all} available pseudo-Schwarzschild potentials to formulate and solve the equations governing the accretion and wind only in terms of flow temperature $T$ and specific angular momentum $λ$ of the flow. Our generalized formalism assures that the shock formation is not just an artifact of a particular type of gravitational potential, rather inclusion of all available black-hole potentials demonstrates a substantially extended zone of parameter space allowing for the possibility of shock formation. We thus arrive at the conclusion that the standing shocks are essential ingredients in rotating, advective accretion flows of isothermal fluid around a non-spinning astrophysical black hole. Types of shocks discussed in this paper may appear to be `bright' because of the huge amount of energy dissipation at shock, and the quick removal of such energy to maintain the isothermality may power the strong X-ray flares recently observed to be emerged from our Galactic centre. The results are discussed in connection to other astrophysical phenomena of related interest, such as the QPO behaviour of galactic black hole candidates. (Abridged).

astro-ph↗
arXiv2003

Role of shocked accretion flows in regulating the QPO of galactic black hole candidates

Using a generalized non-spherical, multi-transonic accretion flow model, we analytically calculate the normalized QPO frequency ${\bar {\bf ν}}_{qpo}$ of galactic black hole candidates in terms of dynamical flow variables and self-consistently study the dependence of ${\bar {\bf ν}}_{qpo}$ on such variables. Our results are in fairly close agreement with the observed QPO frequencies of GRS 1915+105. We find that ${\bar {\bf ν}}_{qpo}$ is quite sensitive to various parameters describing the black hole accretion flow containing dissipative and non-dissipative shock waves. Thus the QPO phenomena is, {\it indeed}, regulated by `shocked' black hole accretion, and, for the first time, we establish a definitive connection between the QPO frequency and the properties of advective BH accretion flows. This information may provide the explanation of some important observations of galactic micro quasars.

astro-ph↗
arXiv2002

Pseudo-Schwarzschild Description of Accretion-Powered Spherical Outflow

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, spherical shock around the central compact object and then we self-consistently connect the accretion-wind solutions to calculate the mass outflow rate $R_{\dot m}$ in terms of minimum number of flow parameters. Also we study the dependence of this rate on various boundary conditions governing the flow.

astro-ph↗
arXiv2002

On the Formation of Accretion-powered Galactic and Extra-galactic Outflows

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 {\em 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 the {\em exact} location from where the outflows are launched.

astro-ph↗
arXiv2002

Generalized Shock Solutions for Hydrodynamic Black Hole Accretion

For the first time, {\it 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 {\it 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.

astro-ph↗
arXiv2002

Accretion powered spherical wind in general relativity

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 $R_{\dot m}$ 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 $R_{\dot m}$, 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.

astro-ph↗
arXiv2002

One Parameter Solution of Spherically Symmetric Accretion in Various Pseudo-Schwarzschild Potentials

In this paper we have solved the hydrodynamic equations governing the spherically symmetric isothermal accretion (wind) onto (away from) compact objects using various pseudo-Schwarzschild potentials.These solutions are essentially one parameter solutions in a sense that all relevant dynamical as well as thermodynamic quantities for such a flow could be obtained (with the assumption of a one-temperature fluid) if {\it 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 depending on the potentials used to study the flow and we have identified these transition zones for flows in various such potentials.

astro-ph↗
arXiv2002

Pseudo-Schwarzschild description of transonic spherical accretion onto compact objects

A number of `modified' Newtonian potentials of various forms are available in the literature which accurately 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 accretion 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 ${\cal E}$ 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 ${\cal E}$ and $γ$.

astro-ph↗
arXiv2002

On Some Transonic Aspects of General Relativistic Spherical Accretion onto Schwarzschild Black Holes

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 {\it 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.

astro-ph↗
arXiv2002

Thermally Driven Outflows from Pair-Plasma Pressure Mediated Shock Surfaces around Schwarzschild Black Holes

Introducing a spherical, steady, self-supported pair-plasma pressure mediated shock surface around a Schwarzschild black hole as the effective physical atmosphere which may be responsible for the generation of astrophysical mass outflows from relativistic quasi-spherical accretion, we calculate the mass outflow rate $R_{\dot m}$ by simultaneously solving the set of equations governing transonic polytropic accretion and isothermal winds. $R_{\dot m}$ is computed in terms of {\it only three} inflow parameters, which, as we believe, has been done for the first time in our work. We then study the dependence of $R_{\dot m}$ 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.

astro-ph↗
arXiv1999

The Impact of Net Culture on Mainstream Societies: a Global Analysis

In this work the impact of the Internet culture on standard mainstream societies has been analyzed. After analytically establishing the fact that the Net can be viewed as a pan-societal superstructure which supports its own distinct culture, an ethnographic analysis is provided to find out the key aspects of this culture. The elements of this culture which have an empowering impacts on the standard mainstream societies, as well as the elements in it which can cause discouraging social effects are then discussed by a global investigation of the present status of various fundamental aspects (e,g, education, economics, politics, entertainment etc) of the mainstream societies as well as their links with the Net culture. Though immensely potential for providing various prominent positive impacts, the key findings of this work indicate that misuse of Internet can create tremendous harm to the members of the mainstream societies by generating a set of morally crippled people as well as a future generation completely void of principles and ethics. This structured diagnostic approach to the social problems caused by the manhandling of Internet leads to a concrete effort of providing the measures that can be taken to enhance or to overcome the supporting and limiting effects of the Net culture respectively with the intent to benefit our society and to protect the teratoidation of certain ethical values.

cs.CY↗