Correlation among QPO frequencies and Quiescence-state Duration in Black Hole Candidate GRS 1915+105

We discover a definite correlation between the frequency of the quasi-periodic oscillations (QPO) in quiescence states and the duration of the quiescence state of the transient X-ray source GRS 1915+105. We find that while the QPO frequency can be explained with the oscillation of shocks in accretion flows, the switching of burst to quiescence states (and vice versa) and their duration can be explained by assuming an outflow from the post-shock region. The duration of the quiescence state is inversely related to the QPO-frequency. We derive this relation. We also find the correlation between the observed low ($\sim 0.001-0.01$Hz) and the intermediate (1-10Hz) QPO frequencies. Our analytical solutions are verified by analyzing several days of public-domain data from RXTE.Comment: Latex, 13 pages with 3 figures; Accepted for Publication in Astrophysical Journal Letter

Satellite observations of thought experiments close to a black hole

Since black holes are `black', methods of their identification must necessarily be indirect. Due to very special boundary condition on the horizon, the advective flow behaves in a particular way, which includes formation of centrifugal pressure dominated boundary layer or CENBOL where much of the infall energy is released and outflows are generated. The observational aspects of black holes must depend on the steady and time-dependent properties of this boundary layer. Several observational results are written down in this review which seem to support the predictions of thought experiments based on this advective accretion/outflow model. In future, when gravitational waves are detected, some other predictions of this model could be tested as well.Comment: Published in Classical and Quantum Gravity, v. 17, No. 12, p. 2427, 200

Simulation of Thick Accretion Disks with Standing Shocks by Smoothed Particle Hydrodynamics

We present results of numerical simulation of inviscid thick accretion disks and wind flows around black holes. We use Smoothed Particle Hydrodynamics (SPH) technique for this purpose. Formation of thick disks are found to be preceded by shock waves travelling away from the centrifugal barrier. For a large range of the parameter space, the travelling shock settles at a distance close to the location obtained by a one-and-a-half dimensional model of inviscid accretion disks. Occasionally, it is observed that accretion processes are aided by the formation of oblique shock waves, particularly in the initial transient phase. The post-shock region (where infall velocity suddenly becomes very small) resembles that of the usual model of thick accretion disk discussed in the literature, though they have considerable turbulence. The flow subsequently becomes supersonic before falling into the black hole. In a large number of cases which we simulate, we find the formation of strong winds which are hot and subsonic when originated from the disk surface very close to the black hole but become supersonic within a few tens of the Schwarzschild radius of the blackhole. In the case of accretion of high angular momentum flow, very little amount of matter is accreted directly onto the black hole. Most of the matter is, however, first squeezed to a small volume close to the black hole, and subsequently expands and is expelled as a strong wind. It is quite possible that this expulsion of matter and the formation of cosmic radio jets is aided by the shock heating in the inner parts of the accretion disks.Comment: LaTeX, 16 pages, Astrophysical Journal (in press

Proteinopathy, oxidative stress and mitochondrial dysfunction: cross talk in alzheimer’s disease and parkinson’s disease

Alzheimer's disease and Parkinson's disease are two common neurodegenerative diseases of the elderly people that have devastating effects in terms of morbidity and mortality. The predominant form of the disease in either case is sporadic with uncertain etiology. The clinical features of Parkinson's disease are primarily motor deficits, while the patients of Alzheimer's disease present with dementia and cognitive impairment. Though neuronal death is a common element in both the disorders, the postmortem histopathology of the brain is very characteristic in each case and different from each other. In terms of molecular pathogenesis, however, both the diseases have a significant commonality, and proteinopathy (abnormal accumulation of misfolded proteins), mitochondrial dysfunction and oxidative stress are the cardinal features in either case. These three damage mechanisms work in concert, reinforcing each other to drive the pathology in the aging brain for both the diseases; very interestingly, the nature of interactions among these three damage mechanisms is very similar in both the diseases, and this review attempts to highlight these aspects. In the case of Alzheimer's disease, the peptide amyloid beta (A beta) is responsible for the proteinopathy, while alpha-synuclein plays a similar role in Parkinson's disease. The expression levels of these two proteins and their aggregation processes are modulated by reactive oxygen radicals and transition metal ions in a similar manner. In turn, these proteins - as oligomers or in aggregated forms - cause mitochondrial impairment by apparently following similar mechanisms. Understanding the common nature of these interactions may, therefore, help us to identify putative neuroprotective strategies that would be beneficial in both the clinical conditions

A thermodynamic basis for prebiotic amino acid synthesis and the nature of the first genetic code

Of the twenty amino acids used in proteins, ten were formed in Miller's atmospheric discharge experiments. The two other major proposed sources of prebiotic amino acid synthesis include formation in hydrothermal vents and delivery to Earth via meteorites. We combine observational and experimental data of amino acid frequencies formed by these diverse mechanisms and show that, regardless of the source, these ten early amino acids can be ranked in order of decreasing abundance in prebiotic contexts. This order can be predicted by thermodynamics. The relative abundances of the early amino acids were most likely reflected in the composition of the first proteins at the time the genetic code originated. The remaining amino acids were incorporated into proteins after pathways for their biochemical synthesis evolved. This is consistent with theories of the evolution of the genetic code by stepwise addition of new amino acids. These are hints that key aspects of early biochemistry may be universal.Comment: 16 pages, 2 tables, 4 figures. Accepted for publication in Astrobiolog

Accretion flow behaviour during the evolution of the Quasi Periodic Oscillation Frequency of XTE J1550-564 in 1998 outburst

Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well-defined correlation among the frequencies of the QPOs over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550-564 obtained during 1998 outburst, and study the systematic drifts during the rising phase from the 1998 September 7 to the 1998 September 19, when the QPO frequency increased monotonically from 81mHz to 13.1Hz. Immediately after that, QPO frequency started to decrease and on the 1998 September 26, the QPO frequency became 2.62Hz. After that, its value remained almost constant. This frequency drift can be modelled satisfactorily with a propagatory oscillating shock solution where the post-shock region behaves as the Comptonized region. Comparing with the nature of a more recent 2005 outburst of another black hole candidate GRO 1655-40, where QPOs disappeared at the end of the rising phase, we conjecture that this so-called `outburst' may not be a full-fledged outburst.Comment: 15 pages, 6 figure

Exotic Bialgebra S03: Representations, Baxterisation and Applications

The exotic bialgebra S03, defined by a solution of the Yang-Baxter equation, which is not a deformation of the trivial, is considered. Its FRT dual algebra $s03_F$ is studied. The Baxterisation of the dual algebra is given in two different parametrisations. The finite-dimensional representations of $s03_F$ are considered. Diagonalisations of the braid matrices are used to yield remarkable insights concerning representations of the L-algebra and to formulate the fusion of finite-dimensional representations. Possible applications are considered, in particular, an exotic eight-vertex model and an integrable spin-chain model.Comment: 24 pages, Latex; V2: revised subsection 4.1, added 9 references, to appear in Annales Henri Poincare in the volume dedicated to D. Arnaudo