Solid state 13C-NMR spectroscopy of some oligocene coals of Assam and Nagaland

Five representative sub-bituminous coals from NE part of India have been analyzed with respect to their distribution of carbon functional groups through ¹³C solid state NMR experiments. The ¹³C Cross Polarised/Magic Angle Spinning NMR spectra of the coal samples were recorded at temperatures of 23°C, 50°C and 70°C respectively and the changes on their structures were interpreted. The NMR spectra of low rank coals were found to exhibit varying degrees of fine structures. The aromaticities of the coal were also determined from the C/H ratios of coal. Some changes in the peak positions along with the formation of simple aromatic units were observed on mild heating.     

Climate‐modulated sequence development in a tropical rift basin during the Late Palaeocene to Early Eocene super greenhouse Earth

Sequence developments in rift basins are considered to be influenced largely by tectonics and to a lesser extent by eustatic sea‐level and climate. Studies indicate that in passive margin basins, climate can mask the effects of tectonics and eustasy by modulating the sediment supply. It is, however, less understood how the sedimentary sequence in rift basins might respond to strong climatic fluctuations where tectonic pulses generate rapid accommodation space. Here a case study has been provided to assess the effect of climate vis à vis sea‐level and tectonics on sequence development in the Cambay rift basin, western India, during the Early Palaeogene (Late Palaeocene to Early Eocene) super greenhouse globe. Facies analysis of this shale–lignite sequence suggests deposition in a lagoon/bay, developed over the Deccan Trap basement. Detailed sequence stratigraphic analysis using basin‐wide representative composite sections, marker lignite seam, event bed and high‐resolution carbon isotope (δ¹³C) chemostratigraphy suggest an overall transgressive motif. Among the three prominent Early Eocene eustatic highstands, only the one at ca 53·7 Ma is expressed by the thickest coal accumulation throughout the basin. Expression of the other sequence stratigraphic surfaces is subdued and can be due to the overall finer grain size of the sediment or local variation in the subsidence rate at different fault‐controlled mini‐basins. Enigmatic presence of a maximum flooding surface coinciding with the 53·7 Ma climate event (Eocene Thermal Maximum 2), manifested by negative carbon isotope excursion, indicates possible influence of climate over and above tectonics in developing the rift sequence. Qualitative rainfall variation assessed using the magnitude of the carbon isotope excursion and pollen abundance show that a relatively dry/low precipitation climatic phase during the Eocene Thermal Maximum 2 hindered the siliclastic supply to the basin. Thus, it has been inferred that climate‐induced high siliciclastic supply possibly enhanced the autocyclic reorganization and hindered the development of the key sequence stratigraphic surfaces across the basin during climate extremes.     

Fresnel zone plate telescopes for X-ray imaging I: experiments with a quasi-parallel beam

Combination of Fresnel Zone Plates (FZP) can make an excellent telescope for imaging in X-rays. We present here the results of our experiments with several pairs of tungsten made Fresnel Zone plates in presence of an X-ray source kept at a distance of about 45 ft. The quasi-parallel beam allowed us to study sources placed on the axis as well as off the axis of the telescope. We present theoretical study of the fringe patterns produced by the zone plates in presence of a quasi-parallel source. We compare the patterns obtained from experiments with those obtained by our Monte-Carlo simulations. The images are also reconstructed by deconvolution from both the patterns. We compare the performance of such a telescope with other X-ray imaging devices used in space-astronomy.     

Unsteady hydromagnetic flow in an annulus with porous Walls when the inner boundary starts moving from rest

Summary The problem of unsteady hydromagnetic flow in a porous annulus when the inner boundary starts moving from rest with a constant longitudinal velocity is considered. For the case of equal Reynolds number and magnetic Reynolds number Laplace transforms of the solutions for the axial components of velocity and magnetic field are obtained in terms of modified Bessel functions. The inversions are effected for the cases of very large and small Reynolds number when the pressure gradient vanishes.     

Unsteady hydromagnetic flow in a porous annulus with time-dependent pressure gradient

Summary Problem of unsteady motion of a conducting viscous incompressible fluid through an annulus with porous walls under an external radial magnetic field has been discussed. Taking the Reynolds number and the magnetic Reynolds number to be equal, exact solution of the problem is obtained in terms of Bessel functions when the motion is due to the time-dependent pressure gradient or the time-dependent velocity of either of the boundaries. Two types of dependence on time are considered; one, exponentially increasing, and the other, exponentially decreasing. Solutions in certain extreme cases are also discussed.     

Walsh spectra of gravity anomalies over some simple sources

Walsh transform of gravity anomalies over a point mass, a horizontal and a vertical line mass have been computed to obtain a cyclic shift invariant differential energy density (DED) function. Quantitative relations between DED spectral characteristics with depth to centroid/top of the source have been established. The effects of profile length, sampling interval, random noise and zero padding have been investigated. Applicability of the proposed method has been evaluated through two field examples.     

Broad-band spectral evolution and temporal variability of IGR J17091-3624 during its 2016 outburst: <Emphasis Type="Italic">SWIFT</Emphasis> and <Emphasis Type="Italic">NuSTAR</Emphasis> results

We report on the 2016 outburst of the transient Galactic Black Hole candidate IGR J17091-3624 based on the observation campaign carried out with SWIFT and NuSTAR. The outburst profile, as observed with SWIFT-XRT, shows a typical ‘q’-shape in the Hardness Intensity Diagram (HID). Based on the spectral and temporal evolution of the different parameters, we are able to identify all the spectral states in the q-profile of HID and the Hardness-RMS diagram (HRD). Both XRT and NuSTAR observations show an evolution of low frequency Quasi periodic oscillations (QPOs) during the low hard and hard intermediate states of the outburst rising phase. We also find mHz QPOs along-with distinct coherent class variabilities (heartbeat oscillations) with different timescales, similar to the \(\rho \)-class (observed in GRS 1915+105). Phenomenological modelling of the broad-band XRT and NuSTAR spectra also reveals the evolution of high energy cut-off and presence of reflection from ionized material during the rising phase of the outburst. Further, we conduct the modelling of X-ray spectra of SWIFT and NuSTAR in 0.5–79 keV to understand the accretion flow dynamics based on two component flow model. From this modelling, we constrain the mass of the source to be in the range of \(10.62\mbox{--}12.33~\mbox{M}_{\odot }\) with 90% confidence, which is consistent with earlier findings.     

Observational aspects of outbursting black hole sources: Evolution of spectro-temporal features and X-ray variability

We report on our attempt to understand the outbursting profile of Galactic Black Hole sources, keeping in mind the evolution of temporal and spectral features during the outburst. We present results of evolution of quasi-periodic oscillations, spectral states and possible connection with jet ejections during the outburst phase. Further, we attempt to connect the observed X-ray variabilities (i.e., ‘class’/‘structured’ variabilities, similar to GRS 1915+105) with spectral states of black hole sources. Towards these studies, we consider three black hole sources that have undergone single (XTE J1859+226), a few (IGR J17091-3624) and many (GX 339-4) outbursts since the start of RXTE era. Finally, we model the broadband energy spectra (3–150 keV) of different spectral states using RXTE and NuSTAR observations. Results are discussed in the context of two-component advective flow model, while constraining the mass of the three black hole sources.