IUCAA Preprints
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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 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 Associated spectral and temporal state transition of the bright ULX NGC 1313 X-1(2010-05-21) Dewangan, Gulab Chand; et al.Stellar mass black hole X-ray binaries exhibit X-ray spectral states which also have distinct and characteristic temporal properties. These states are believed to correspond to different accretion disc geometries. We present analysis of two XMM-Newton observations of the Ultra-Luminous X-ray source (ULX) NGC 1313 X-1, which reveal that the system was in two different spectral states. While spectral variations have been observed in this source before, this data provides clear evidence that the spectral states also have distinct temporal properties. With a count rate of ~ 1.5 counts/s and a fractional variability amplitude of ~ 15%, the ULX was in a high flux and strongly variable state in March 2006. In October 2006, the count rate of the ULX had reduced by a factor of ~ 2 and the spectral shape was distinctly different with the presence of a soft component. No strong variability was detected during this low flux state with an upper limit on the amplitude < 3%. Moreover, the spectral properties of the two states implies that the accretion disc geometry was different for them. The low flux state is consistent with a model where a standard accretion disc is truncated at a ra- dius of ~ 17 Schwarzschild radius around a ~ 200M⊙ black hole. The inner hot region Comptonizes photons from the outer disc to give the primary spectral component. The spectrum of the high flux state is not compatible with such a geometry. Instead, it is consistent with a model where a hot corona covers a cold accretion disc and Comptonizes the disc photons. The variability as a function of energy is also shown to be consistent with the corona model. Despite these broad analogies with Galactic black hole systems, the spectral nature of the ULX is distinct in having a lower temperature (~ 2 keV) of the hot Comptonizing plasma and higher optical depth (~ 15) than what is observed for the Galactic ones.