IUCAA Preprints
Permanent URI for this communityhttp://localhost:4000/handle/11007/149
Browse
10 results
Search Results
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 Effect of dissipative corona on the structure and stability of cold optically thick accretion disks at high accretion rates(2002-03-20) Misra, Ranjeev; Taam, Ronald E.The vertical structure of optically thick accretion disks is investigated in the two-zone approximation. The disk is divided into an underlying disk and a corona, where the latter is defined as the upper surface layers for which the effective optical depth is unity. It is found that a significant part of the accretion flow (or dissipation rate) can take place in the corona if the scale height of the magnetic field is larger than that of the disk. The presence of such a dissipative corona leads to a modification in the topology of local disk solutions. For example, these solutions are found from local stability analysis to be both secularly and thermally stable, for accretion rates which are a factor ≈ four higher than those inferred from the stability of standard disk solutions. Thus, the applicability of optically thick disks with dissipative coronas are not as restrictive as disks without such coronas and can provide an attractive explanation for the origin of the soft spectral component observed in black hole X-ray binary systems.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 predictionItem Angular momentum transport in quasi-Keplerian accretion disks(2011-07-06) Subramanian, Prasad; Pujari, B. S.; Becker, Peter A.We reexamine arguments advanced by Hayashi &Matsuda (2001), who claim that several simple, physically motivated deriva- tions based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the na- ture of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expres- sion for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The deriva- tion is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.Item Secular instability in quasi-viscous disc accretion(2007-07-12) Bhattacharjee, Jayanta K.; Ray, Arnab K.A first-order correction in the -viscosity parameter of Shakura& Sunyaev has been introduced in the standard inviscid and thin accretion disc. A linearised time-dependent perturbative study of the stationary solutions of this “quasi-viscous” disc leads to the development of a secular instability on large spatial scales. This qualitative feature is equally manifest for two different types of perturbative treatment — a standing wave on subsonic scales, as well as a radially propagating wave. Stability of the flow is restored when viscosity disappears.Item 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.Item Critical properties of spherically symmetric accretion in a fractal medium(2007-07-18) Roy, Nirupam; Ray, Arnab K.Spherically symmetric transonic accretion of a fractal medium has been studied in both the stationary and the dynamic regimes. The stationary transonic solution is greatly sensitive to infinitesimal deviations in the outer boundary condition, but the flow becomes transonic and stable, when its evolution is followed through time. The evolution towards transonicity is more pronounced for a fractal medium than what is it for a continuum. The dynamic approach also shows that there is a remarkable closeness between an equation of motion for a perturbation in the flow, and the metric of an analogue acoustic black hole. The stationary inflow solutions of a fractal medium are as much stable under the influence of linearised perturbations, as they are for the fluid continuum.Item 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.Item Axisymmetric black hole accretion in the Kerr metric as an autonomous dynamical system(2007-02-07) Goswami, Sanghamitra; Khan, Saba Nashreen; Ray, Arnab K.; et al.In a stationary, general relativistic, axisymmetric, inviscid and rotational accretion flow, described within the Kerr geometric framework, transonicity has been examined by setting up the governing equations of the flow as a first-order autonomous dynamical system. The consequent linearised analysis of the critical points of the flow leads to a comprehensive mathematical prescription for classifying these points, showing that the only possibilities are saddle points and centre-type points for all ranges of values of the fixed flow parameters. The spin parameter of the black hole influences the multitransonic character of the flow, as well as some of its specific critical properties. The special case of a flow in the space-time of a non-rotating black hole, characterised by the Schwarzschild metric, has also been studied for comparison and the conclusions are compatible with what has been seen for the Kerr geometric case.Item X-ray emission from active galactic nuclei with intermediate mass black holes(2008-12) Dewangan, Gulab ChandWe present a systematic X-ray study of eight active galactic nuclei (AGNs) with intermediate mass black holes (MBH ∼ 8 − 95 × 104 M⊙) based on 12 XMM-Newton observations. The sample includes the two prototype AGNs in this class – NGC 4395 and POX 52 and six other AGNs discovered with the Sloan Digitized Sky Survey. These AGNs show some of the strongest X-ray variability with the normalized excess variances being the largest and the power density break time scales being the shortest observed among radio-quiet AGNs. The excess variance – luminosity correlation appears to depend on both the BH mass and the Eddington luminosity ratio. The break time scale – black hole mass relations for AGN with IMBHs are consistent with that observed for massive AGNs. We find that the FWHM of the Hβ/Hα line is uncorrelated with the BH mass, but shows strong anticorrelation with the Eddington luminosity ratio. Four AGNs show clear evidence for soft X-ray excess emission (kTin ∼ 150 −200 eV). X-ray spectra of three other AGNs are consistent with the presence of the soft excess emission. NGC 4395 with lowest L/LEdd lacks the soft excess emission. Evidently small black mass is not the primary driver of strong soft X-ray excess emission from AGNs. The X-ray spectral properties and optical-to-X-ray spectral energy distributions of these AGNs are similar to those of Seyfert 1 galaxies. The observed X-ray/UV properties of AGNs with IMBHs are consistent with these AGNs being low mass extension of more massive AGNs; those with high Eddington luminosity ratio looking more like narrow-line Seyfert 1s while those with low L/LEdd looking more like broad-line Seyfert 1s.