Browsing by Author "Misra, Ranjeev"

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Active Galactic Nuclei
(2013-07-16) Dewangan, Gulab Chand; Misra, Ranjeev
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.
Non subjective approach to the GP algorithm for analysing noisy time series
(2006-01-10) Harikrishnan, K. P.; Misra, Ranjeev; Ambika, G.; et al.
We present an adaptation of the standard Grassberger-Proccacia (GP) algorithm for estimating the Correlation Dimension of a time series in a non subjective manner. The validity and accuracy of this approach is tested using di erent types of time series, such as, those from standard chaotic systems, pure white and colored noise and chaotic systems added with noise. The e ectiveness of the scheme in analysing noisy time series, particularly those involving colored noise, is investigated. An interesting result we have obtained is that, for the same percentage of noise addition, data with colored noise is more distinguishable from the corresponding surrogates, than data with white noise. As examples for real life applications, analysis of data from an astrophysical X-ray object and human brain EEG, are presented.
Non-linear behaviour of the black hole system GRS 1915+ 105
(2006-02-05) Misra, Ranjeev; Harikrishnan, K. P.; Ambika, G.; et al.
Using non-lin~ar ti?Ie seri.e~ analysis, along with surrogate data analysis, it is shown that the various types of lon~ t~rI? varla~lbty exhibited by the black hole system GRS 1915+105, can be explained in terms of a ~et~rmlmstlc.n?n-bnear system with some inherent stochastic noise. Evidence is provided for a non-linear limit cycle ongm of one of.the low f~eque~cy QPO detected in the source, while some other types of variability could. be due ~oan underlying low dimensional chaotic system. These results imply that the partial differential equanons which govern the magneto- ~drodynamic flow of the inner accretion disk, can be approximated by a by a small number ( ~ 3 - 5) of non-linear but ordinary differential equations. While this analysis does not reveal the exact nature of these approximate equations, they may be obtained in the future, after results of magneto-hydrodynamic simulation of realistic accretion disks become available.
Pseudo-Newtonian Potentials to Describe the Temporal Effects on Relativistic Accretion Disks around Rotating Black Holes and Neutron Stars
(2002-01-02) Mukhopadhyay, B.; Misra, Ranjeev
Two pseudo-Newtonian potentials, which approximate the angular and epicyclic frequencies of the relativistic accretion disk around rotating (and counter rotating) compact objects, are presented. One of them, the Logarithmically Modiﬁed Potential, is a better approximation for the frequencies while the other, the Second-order Expanded potential, also reproduces the speciﬁc energy for circular orbits in close agreement with the General Relativistic values. These potentials may be included in time dependent hydrodynamical simulations to study the temporal behavior of such accretion disks
Soft time lags in the X-ray emission of mrk 1040
(2011-01-30) Tripathi, Shruti; Misra, Ranjeev; Dewangan, Gulab Chand; et al.
Temporal analysis of X-ray binaries and Active Galactic Nuclei have shown that hard X-rays react to variation of soft ones after a time delay. The opposite trend, or soft lag, has only been seen in a few rare Quasi-periodic Oscillations in X-ray binaries and recently for the AGN, 1H 0707-495, on short timescales of ∼ 103 secs. Here, we report analysis of a XMM-Newton observation of Mrk 1040, which reveals that on the dominant variability timescale of ∼ 104 secs, the source seems to exhibit soft lags. If the lags are frequency independent, they could be due to reverberation eﬀects of a relativistically blurred reﬂection component responding to a varying continuum. Alternatively, they could be due to Comptonization delays in the case when high energy photons impinge back on the soft photon source. Both models can be veriﬁed and their parameters tightly constrained, because they will need to predict the photon spectrum, the r.m.s variability and time lag as a function of energy. A successful application of either model will provide unprecedented information on the radiative process, geometry and more importantly the size of the system, which in turn may provide stringent test of strong general relativistic effects.
Spectral variability of IRAS 18325-5926 and Contraints on the Geometry of the Scattering Medium
(IUCAA, 2015-02) Tripathi, Shruti; Misra, Ranjeev; Dewangan, Gulab Chand
Timing Properties of Accreting Black Hole Systems
(2011-01-29) Misra, Ranjeev