Santabrata Das

Properties of accretion shock waves in viscous flows around black holes

Accretion flows having low angular momentum and low viscosity can have standing shock waves. These shocks arise because of the presence of multiple sonic points in the flow. We study the region of the parameter space in which multiple sonic points occur in viscous flows in the absence of cooling. We also separate the parameter space into regions allowing steady shocks and oscillating shocks. We quantify the nature of two critical viscosities which separate the flow topologies. The post-shock region being hotter, it emits harder X-rays and oscillating shocks cause oscillating X-ray intensities giving rise to quasi-periodic oscillations. We show that with the increase in viscosity parameter, the shock always moves closer to the black hole. This implies an enhancement of the quasi-periodic oscillation frequency as viscosity is increased.

Computation of outflow rates from accretion disks around black holes

We self-consistently estimate the outflow rate from the accretion rates of an accretion disk around a black hole in which both the Keplerian and the sub-Keplerian matter flows simultaneously. While Keplerian matter supplies soft-photons, hot sub-Keplerian matter supplies thermal electrons. The temperature of the hot electrons is decided by the degree of inverse Comptonization of the soft photons. If we consider only thermally-driven flows from the centrifugal pressure-supported boundary layer around a black hole, we find that when the thermal electrons are cooled down, either because of the absence of the boundary layer (low compression ratio), or when the surface of the boundary layer is formed very far away, the outflow rate is negligible. For an intermediate size of this boundary layer the outflow rate is maximal. Since the temperature of the thermal electrons also decides the spectral state of a black hole, we predict that the outflow rate should be directly related to the spectral state.

STANDING SHOCKS AROUND BLACK HOLES: AN ANALYTICAL STUDY

We compute locations of sonic points and standing shock waves in a thin, axisymmetric, adiabatic flow around a Schwarzschild black hole. We use a completely analytical method to achieve our goal. Our results are compared with those obtained numerically, and good agreement is seen. Our results prove the existence of shocks in centrifugal pressure-dominated flows. We indicate how our results can be used to obtain spectral properties and frequencies of shock oscillations, which may be directly related to the quasi-periodic oscillations of hard X-rays.

PROPERTIES OF ACCRETION SHOCKS IN VISCOUS FLOWS WITH COOLING EFFECTS

Low angular momentum accretion flows can have standing and oscillating shock waves. We study the region of the parameter space in which multiple sonic points occur in vis-cous flows in presence of various cooling effects such as bremsstrahlung and Comptoniza-tion. We also quantify the parameter space in which shocks are steady or oscillating. We find that cooling induces effects opposite to heating by viscosity even in modifying the topology of the solutions, though one can never be exactly balanced by the other due to their dissimilar dependence on dynamic and thermodynamic parameters. We show that beyond a critical value of cooling, the flow ceases to contain a shock wave.

Radiatively driven electron—positron jets from two-component accretion flows

Matter accreting on to black holes has long been known to have standing or oscillating shock waves. The post-shock matter puffs up in the form of a torus, which intercepts soft photons from the outer Keplerian disc and inverse Comptonizes to produce hard photons. The post-shock region also produces jets. We study the interaction of both hard photons and soft photons, with on-axis electron‐positron jets. We show that the radiation from post-shock torus accelerates the flow to relativistic velocities, while that from the Keplerian disc has marginal effect. We also show that the velocity at infinity or the terminal velocity ϑ depends on the shock location in the disc.

Model dependence of transonic properties of accretion flows around black holes

We analytically study how the behaviour of accretion flows changes when the flow model is varied. We study the transonic properties of the conical flow, a flow of constant height and a flow in vertical equilibrium, and show that all these models are basically identical, provided that the polytropic constant is suitably changed from one model to another. We show that this behaviour is extendible even when standing shocks are produced in the flow. The parameter space where shocks are produced remains roughly identical in all these models when the same transformation among the polytropic indices is used. We present applications of these findings.

Estimation of mass outflow rates from dissipative accretion disc around rotating black holes

We study the properties of the dissipative accretion flow around rotating black holes in the presence of mass loss. We obtain a complete set of global inflow-outflowsolutions in the steady state by solving the underlying conservation equations self-consistently. We observe that global inflow-outflow solutions are not the isolated solution, instead such solutions are possible for wide range of inflow parameters. Accordingly, we identify the boundary of the parameter space for outflows, spanned by the angular momentum (lambda(in)) and the energy (epsilon(in)) at the inner sonic point (chi(in)), as a function of the dissipation parameters and find that parameter space gradually shrinks with the increase of dissipation rates. Further, we examine the properties of the outflow rate R-m (defined as the ratio of the outflow-to-inflow mass flux) and ascertain that dissipative processes play a decisive role in determining the outflow rates. We calculate the limits on the maximum outflow rate (R-m(max)) in terms of viscosity parameter (a) as well as the black hole spin (a(k)) and obtain the limiting range as 3 per cent = 0.57.

Parameter space study of the magnetohydrodynamic accretion flows around compact objects

We solve the magnetohydrodynamic (MHD) equations governing axisymmetric flows around compact objects and found all possible classes of solutions for non-relativistic adiabatic accretion flows. We divide the parameter space in terms of these classes. We study the possibility of the formation of the MHD shock waves and show how the strength of the shocks depends on the flow parameters. We also show regions of the parameter space where the shock conditions are not satisfied and therefore the shocks may oscillate. These solutions are astrophysically interesting as they could give rise to quasi-periodic oscillations seen in hard X-rays.

Properties of magnetically supported dissipative accretion flow around black holes with cooling effects

We investigate the global structure of the advection dominated accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. We consider synchrotron radiative process as an effective cooling mechanism active in the flow. With this, we obtain the global transonic accretion solutions by exploring the variety of boundary conditions and dissipation parameters, namely accretion rate (

Properties of two-temperature dissipative accretion flow around black holes

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