62 research outputs found

    Noise-induced Regime Shifts: A Quantitative Characterization

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    Diverse complex dynamical systems are known to exhibit abrupt regime shifts at bifurcation points of the saddle-node type. The dynamics of most of these systems, however, have a stochastic component resulting in noise driven regime shifts even if the system is away from the bifurcation points. In this paper, we propose a new quantitative measure, namely, the propensity transition point as an indicator of stochastic regime shifts. The concepts and the methodology are illustrated for the one-variable May model, a well-known model in ecology and the genetic toggle, a two-variable model of a simple genetic circuit. The general applicability and usefulness of the method for the analysis of regime shifts is further demonstrated in the case of the mycobacterial switch to persistence for which experimental data are available.Comment: 10 Pages, 9 figures, revtex4-1, published versio

    Search for coronal temperature variation in Seyfert galaxies

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    While the temperature of the X-ray corona (kTe\rm{kT_e}) in active galactic nuclei (AGN) are known for many sources, its variation, if any, is limited to a handful of objects. This is in part due to the requirement of good signal-to-noise X-ray spectra covering a wide range of energies. We present here results on the X-ray spectral analysis of 18 Seyferts, having more than one epoch of observations to look for variation in kTe\rm{kT_e}. The data for a total of 52 epochs on these 18 AGN were taken from observations carried out by NuSTAR in the 3-79 keV energy band. From phenomenological and physical model fits to the multi-epoch data on these 18 sources from {\it NuSTAR}, we could constrain the cut-off energy (EcutE_{cut}) in a large fraction of the sources. Also, from Comptonized model fits, we could obtain kTe\rm{kT_e} for our sample. Of the 18 sources, at the 90 per cent confidence level, evidence for variation in kTe\rm{kT_e} was found for only one source, namely MCG+08-11-011. For this source, between two epochs, separated by about five years, we found kTe\rm{kT_e} to decrease from 5716+29^{+29}_{-16} keV to 307+11^{+11}_{-7} keV. During the same period, the flux decreased from (12.60 to 14.02) ×\times 1011^{-11} erg cm2^{-2} s1^{-1} and the optical depth increased from 1.68 to 2.73. We thus found a positive correlation between flux and coronal temperature with a reduction of about 40 per cent in optical depth. Our observations tend to favour the vertically outflowing corona scenario for the observed variation in kTe\rm{kT_e} in MCG+08-11-011.Comment: 17 pages, 7 figuers, accepted for publication in MNRAS-M

    Thiosemicarbazone complexes of the platinum metals. A story of variable coordination modes

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    Salicylaldehyde thiosemicarbazone (H2saltsc) reacts with [M(PPh3)3X2] (M = Ru, Os; X = Cl, Br) to afford complexes of type [M(PPh3)2(Hsaltsc)2], in which the salicylaldehyde thiosemicarbazone ligand is coordinated to the metal as a bidentate N,S-donor forming a four-membered chelate ring. Reaction of benzaldehyde thiosemicarbazones (Hbztsc-R) with [M(PPh3)3X2] also affords complexes of similar type, viz. [M(PPh3)2(bztsc-R)2], in which the benzaldehyde thiosemicarbazones have also been found to coordinate the metal as a bidentate N,S-donor forming a four-membered chelate ring as before. Reaction of the Hbztsc-R ligands has also been carried out with [M(bpy)2X2] (M = Ru, Os; X = Cl, Br), which has afforded complexes of type [M(bpy)2(bztsc-R)]+, which have been isolated as perchlorate salts. Coordination mode of bztsc-R has been found to be the same as before. Structure of the Hbztsc-OMe ligand has been determined and some molecular modelling studies have been carried out determine the reason for the observed mode of coordination. Reaction of acetone thiosemicarbazone (Hactsc) has then been carried out with [M(bpy)2X2] to afford the [M(bpy)2(actsc)]ClO4complexes, in which the actsc ligand coordinates the metal as a bidentate N,S-donorformingafive-membered chelate ring. Reaction of H2saltsc has been carried out with [Ru(bpy)2Cl2] to prepare the [Ru(bpy)2(Hsaltsc)]ClO4 complex, which has then been reacted with one equivalent of nickel perchlorate to afford an octanuclear complex of type [Ru(bpy)2(saltsc-H)4Ni4](ClO4)4

    QSPR Modeling of Odor Threshold of Aliphatic Alcohols Using Extended Topochemical Atom (ETA) Indices

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    The present work establishes a quantitative structure-property relationship (QSPR) between top¬ochemical features and odor threshold (OT) of aliphatic alcohols. A data set of 53 aliphatic alcohols was chosen for the analysis employing different chemometric techniques, among which, genetic function ap¬proximation with spline option (GFA-spline) showed the most acceptable results in terms of internal and external validation metric values. The extended topochemical atom (ETA) indices, developed by the pre¬sent authors’ group, were considered as descriptors for model development. Additionally, selected non-ETA descriptors were also tried for model development. It was observed that the models with ETA indi¬ces significantly surpass the predictive ability of the models developed using other descriptors. The final model suggests that molecular branching and electronic parameters significantly influence the odor poten-cy of the molecules. Additionally, increased lipophilicity and reduced electronegativity increase the odor-ant property. The model thus developed may effectively be used for prediction of odor threshold of any untested aliphatic alcohols. (doi: 10.5562/cca2284
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