arXiv subjects

Sankhasubhra Nag

Publications and source records attributed to Sankhasubhra Nag.

16 recordsLinked to original sources

arXiv2026

Model-Dependent Galactic Environment Effects on Multi-Transonic Accretion and Emergent Acoustic Gravity around Kerr Black Holes

The hydrodynamics of low angular momentum, multi-transonic, axisymmetric, inviscid accretion flow onto a rotating black hole in presence of a galactic environment has been systematically investigated in detail using three standard disc geometries and two thermodynamic equations of state, within the post-Newtonian framework with a pseudo-potential. In addition to the influence of a centrally located black hole, this study incorporates those due to a multi-component galactic environment -- stellar matter, dark matter, and hot gas. Our analysis reveals that the extent of additional influence of the galactic environment on accretion flow depends severely on the disc model chosen. This effect is particularly pronounced in the vertical equilibrium disc model, whereas it is extremely mild for other two geometries. The same is observed for location of sonic points, parameters related to multi-transonicity and shock formation, and also for the emergent analogue surface gravity calculated. Another observation is that, among different components of galactic environment, contribution of dark matter is most dominant, followed by a mild one due to hot gas; while that due to stellar matter is even milder in determining the flow. The stationary flow characteristics are analysed using semi-analytical numerical methods including the method analogous to critical point analysis used in dynamical systems. Additionally, a time-dependent linear perturbation analysis ensures the stability of stationary accretion flow in all cases, and hence the corresponding acoustic metric and acoustic surface gravity are derived.

astro-ph.HE↗

arXiv2026

Transonic accretion and the analogue gravity in multi-component elliptical galaxies hosting pseudo-Schwarzschild black holes

Low-angular-momentum, axisymmetric, inviscid accretion flows onto a black hole have been studied using the vertical equilibrium disc model, considering multiple pseudo-Schwarzschild potentials and two thermodynamic equations of state. A multi-component galactic potential-representing stellar, dark matter, and hot-gas contributions-is incorporated to assess environmental effects on the accretion dynamics. In our earlier work, it is found that the effect of multi-component galactic potential on the accretion flow onto a rotating black hole under similar framework of analysis, significantly varies over different standard disc models, being most pronounced in the vertical equilibrium (VE) disc model. Thus it may be interesting to find whether such variation occur for different choices of pseudo potentials too. To begin with, in this work we consider accretion flow onto a non-rotating blackhole with VE geometry. Through the analysis of transonic behaviour and eigenvalue-based critical point classification, we demonstrate that, for all selected black hole potentials, the galactic potential profoundly influences the locations of critical points, the shock-allowed parameter space, shock-location, shock-driven flow variables, and acoustic surface gravity.

astro-ph.HE↗

arXiv2024

Dynamic magnetic response in ABA type trilayered systems and compensation phenomenon

Dynamic magnetic response in a trilayered structure with non-equivalent layers (ABA type) has been studied with Monte Carlo simulation using Metropolis algorithm. In each layer, ferromagnetic (FM) nearest neighbour Ising interactions are present along with antiferromagnetic (AFM) nearest neighbour coupling across different layers. The system is studied under a harmonically oscillating external magnetic field. It is revealed that along with dynamic phase transition (DPT), compensation phenomenon emerges in this system under dynamic scenario too. This feature in dynamic case is unique for such trilayered systems only, in contrast to the bulk system reported earlier. The temporal behaviour of the magnetisation of each individual layer shows that different magnetic response of the non-equivalent layers results into such dynamic compensation phenomenon. The difference in response also results into warping of the dynamic hysteresis loops, under various external parameter values, such as amplitude of the oscillating field and temperature.

cond-mat.stat-mech↗

arXiv2018

Effective sound speed in relativistic accretion discs around Schwarzschild black holes

For low angular momentum axially symmetric accretion flow maintained in hydrostatic equilibrium along the vertical direction, the value of the Mach number at the critical points deviates from unity, resulting in the non-isomorphism of the critical and the sonic points. This introduces several undesirable complexities while analytically dealing with the stationary integral accretion solutions and the corresponding phase portraits. We propose that introduction of an effective dynamical sound speed may resolve the issue in an elegant way. We linear perturb the full spacetime-dependent general relativistic Euler and the continuity equations governing the structure and the dynamics of accretion disc in vertical equilibrium around Schwarzschild black holes and identify the sonic metric embedded within the stationary background flow. Such metric describes the propagation of the linear acoustic perturbation inside the accretion flow. We construct the wave equation corresponding to that acoustic perturbation and find the speed of propagation of such perturbation. We finally show that the ordinary thermodynamic sound speed should be substituted by the speed of propagation of the linear acoustic wave which has been obtained through the dynamical perturbation. Such substitution will make the value of Mach number at the critical point to be equal to unity. Use of the aforementioned effective sound speed will lead to a modified stationary disc structure where the critical and the sonic points will be identical.

astro-ph.HE↗

arXiv2018

Second Order Perturbative Effects on the Acoustic Geometry

In this work, we have studied the effect of higher order perturbations, particularly the second order in details, on the sonic horizon. We have considered two different schemes of perturbations which are velocity potential perturbation and mass acceleration rate perturbation. These two schemes give us qualitatively similar behaviour. We have found that the analogue gravity formalism also holds for the higher order perturbations.

gr-qc↗

arXiv2017

Axially Symmetric Accretion of Fractal Medium onto Rotating Black Holes and the emergence of the Acoustic Manifold

For three different geometric configurations of matter and two different thermodynamic equations of state, low angular momentum, multi-transonic, axially symmetric accretion flow of matter having fractional dimension of mass distribution onto a rotating black hole has been studied by employing certain post-Newtonian pseudo-Kerr black hole potential. Such task has been accomplished mathematically by mapping the fractal nature of accreted medium onto its continuum counterpart. The difference between spin dependence of accretion dynamics of the fractal medium and the continuous medium has been highlighted. By employing a time dependent linear perturbation scheme, it has been demonstrated that accretion of matter with fractional dimension of density distribution can be considered as a natural example of classical analogue model. The corresponding acoustic surface gravity has been estimated in terms of accretion variables. The value of the surface gravity changes as the accreted matter makes a transition from its fractal nature to the corresponding continuum distribution.

astro-ph.HE↗

arXiv2016

Influence of the black hole spin on the chaotic particle dynamics within a dipolar halo

We investigate the role of the spin angular momentum of astrophysical black holes in controlling the special relativistic chaotic dynamics of test particles moving under the influence of a post-Newtonian pseudo-Kerr black hole potential, along with a perturbative potential created by a asymmetrically placed (dipolar) halo. Proposing a Lyapunov-like exponent to be the effective measure of the degree of chaos observed in the system under consideration, it has been found that black hole spin anti-correlates with the degree of chaos for the aforementioned dynamics. Our findings have been explained applying the general principles of dynamical systems analysis.

astro-ph.HE↗

arXiv2015

Transonic behaviour and stability analysis of quasi-viscous black hole accretion

Analytical studies of black hole accretion usually presumes the stability of the stationary transonic configuration. Various authors in the past several decades demonstrated the validity of such an assumption for inviscid hydrodynamic flow. Inviscid approximation is a reasonable approach for low angular momentum advection dominated flow in connection to certain supermassive black holes at the centres of the galaxies (including our own) fed from a number of stellar donors. Introduction of a weak viscosity, as a first order linear correction involving the viscosity parameter, however, may sometimes provide a more detail understanding of the observed black hole spectra. The transonic behaviour of the stationary solutions have been studied for the aforementioned quasi-viscous accretion for all possible geometric configurations of axisymmetric flow. For a sufficiently low range of the viscosity parameter, transonic solutions containing one or three critical points have been found for allowed ranges in the astrophysical parameters under the post-Newtonian pseudo-Schwarzschild scheme. With the introduction of such viscosity parameter, the only feasible critical points are of saddle and spiral types in contrary to the inviscid case where centre type points were formed instead of spiral ones. Introduction of linear perturbations on stationary flow solutions and their time evolution in both standing and radially propagating wave forms have been examined (completely analytically) in detail. Our analysis shows that similar kind of secular instability exists in all the considered disk models at large distance in the asymptotic limit, however, the model itself is valid only within a certain length scale and the disks sustain within that length scale only for at least a considerable time scale.

astro-ph.HE↗

arXiv2015

Model dependence of the multi-transonic behavior, stability properties and corresponding acoustic geometry for accretion onto a spinning black hole

Multi-transonic accretion for a spinning black hole has been compared among different disc geometries within post Newtonian pseudo potential framework. The variation of stationary shock characteristics with black hole spin has been studied in details for all the disc models and compared for adiabatic as well as for isothermal scenario. The variations of surface gravity with spin for all these cases have also been investigated.

astro-ph.HE↗

arXiv2014

Black Hole spin dependence of general relativistic multi-transonic accretion close to the horizon

We introduce a novel formalism to investigate the role of the spin angular momentum of astrophysical black holes in influencing the behaviour of low angular momentum general relativistic accretion. We propose a metric independent analysis of axisymmetric general relativistic flow, and consequently formulate the space and time dependent equations describing the general relativistic hydrodynamic accretion flow in the Kerr metric. The associated stationary critical solutions for such flow equations are provided and the stability of the stationary transonic configuration is examined using an elegant linear perturbation technique. We examine the properties of infalling material for both prograde and retrograde accretion as a function of the Kerr parameter at extremely close proximity to the event horizon. Our formalism can be used to identify a new spectral signature of black hole spin, and has the potential of performing the black hole shadow imaging corresponding to the low angular momentum accretion flow.

astro-ph.HE↗

arXiv2014

Development of Secular Instability in Different Disc Models of Black Hole Accretion

Analytical treatment of black hole accretion generally presumes the stability of the stationary configuration. Various authors in the past several decades demonstrated the validity of such an assumption for inviscid hydrodynamic flow. Inviscid assumption is a reasonable approximation for low angular^M momentum advection dominated flow in connection to certain^M supermassive black holes at the^M centres of the galaxies (including our own) fed from a number of stellar donors.^M Introduction of a weak viscosity, however, may sometimes provide a more detail understanding of the observed spectrum. Recently it has been demonstrated that introduction of small amount of viscosity in the form of quasi-viscous flow makes a stationary accretion disc -- where the geometric configuration of matter is described by axisymmetric flow in hydrostatic equilibrium -- unstable. We perform similar analysis for other disc models (for all three possible geometric configurations of matter) for quasi-viscous models under the post-Newtonian scheme. We introduced perturbations on the stationary flow solution particularly in standing wave form and studied their time evolution to observe whether they grow with time. Our analysis shows that same sort of secular instability exists in other disc models too. We further argued that with sufficiently low value of viscosity in the realistic astrophysical perspective, the instability does not effectively jeopardize the stationary condition.

astro-ph.HE↗

arXiv2012

The role of axisymmetric flow configuration in the estimation of the analogue surface gravity and related Hawking like temperature

For axially symmetric flow of dissipationless inhomogeneous fluid onto a non rotating astrophysical black hole under the influence of a generalized pseudo-Schwarzschild gravitational potential, we investigate the influence of the background flow configuration on determining the salient features of the corresponding acoustic geometry. The acoustic horizon for the aforementioned flow structure has been located and the corresponding acoustic surface gravity $κ$ as well as the associated analogue Hawking temperature $T_{\rm AH}$ has been calculated {\it analytically}. The dependence of $κ$ on the flow geometry as well as on the nature of the back ground black hole space time (manifested through the nature of the pseudo-Schwarzschild potential used) has been discussed. Dependence of the value of $κ$ on various initial boundary conditions governing the dynamic and the thermodynamic properties of the background fluid flow has also been studied.

gr-qc↗

arXiv2012

An Analytical Study on the Multi-critical Behaviour and Related Bifurcation Phenomena for Relativistic Black Hole Accretion

We apply the theory of algebraic polynomials to analytically study the transonic properties of general relativistic hydrodynamic axisymmetric accretion onto non-rotating astrophysical black holes. For such accretion phenomena, the conserved specific energy of the flow, which turns out to be one of the two first integrals of motion in the system studied, can be expressed as a 8$^{th}$ degree polynomial of the critical point of the flow configuration. We then construct the corresponding Sturm's chain algorithm to calculate the number of real roots lying within the astrophysically relevant domain of $\mathbb{R}$. This allows, for the first time in literature, to {\it analytically} find out the maximum number of physically acceptable solution an accretion flow with certain geometric configuration, space-time metric, and equation of state can have, and thus to investigate its multi-critical properties {\it completely analytically}, for accretion flow in which the location of the critical points can not be computed without taking recourse to the numerical scheme. This work can further be generalized to analytically calculate the maximal number of equilibrium points certain autonomous dynamical system can have in general. We also demonstrate how the transition from a mono-critical to multi-critical (or vice versa) flow configuration can be realized through the saddle-centre bifurcation phenomena using certain techniques of the catastrophe theory.

gr-qc↗

arXiv2008

Mixing and Decoherence to Nearest Separable States

We consider a class of entangled states of a quantum system (S) and a second system (A) where pure states of the former are correlated with mixed states of the latter, and work out the entanglement measure with reference to the nearest separable state. Such `pure-mixed' entanglement is expected when the system S interacts with a macroscopic measuring apparatus in a quantum measurement, where the quantum correlation is destroyed in the process of environment-induced decoherence whereafter only the classical correlation between S and A remains, the latter being large compared to the former. We present numerical evidence that the entangled S-A state drifts towards the nearest separable state through decoherence, with an additional tendency of equimixing among relevant groups of apparatus states.

quant-ph↗

arXiv2004

Quantum Chaos: Reduced Density Matrix Fluctuations in Coupled Systems

Following a recent work (briefly reviewed below) we consider temporal fluctuations in the reduced density matrix elements for a coupled system involving a pair of kicked rotors as also one made up of a pair of Harper Hamiltonians. These dynamical fluctuations are found to constitute a reliable indicator of the degree of chaos in the quantum dynamics, and are related to stationary features like the eigenvalue and eigenvector distributions of the system under consideration. A brief comparison is made with the evolution of the reduced distribution function in the classical phase space.

quant-ph↗

arXiv2001

Entropy production due to coupling to a heat bath in the kicked rotor problem

Considering a kicked rotor coupled to a model heat bath both the classical and quantum entropy productions are calculated exactly. Starting with an initial wave packet, the von Neuman entropy as a function of time is determined from the reduced density matrix while the Liouville evolution of the corresponding Husimi distribution provides us with the classical entropy. It is found that both these entropies agree reasonably satisfying the same asymptotic growth law and more importantly both are proportional to the classical Liapounov exponent.

quant-ph↗