Crystal structure of bis-(4-meth-oxy-phenyl) malonate.

Open AccessThe complete mol-ecule of the title compound, C17H16O6, is generated by crystallographic twofold symmetry, with the central methyl-ene C atom lying on the rotation axis. The carbonyl O atom is disordered over two adjacent positions in a 0.63 (3):0.37 (3) ratio and the dihedral angle between the benzene rings in the two halves of the mol-ecule is 79.31 (12)°. In the crystal, mol-ecules are connected by C-H⋯O hydrogen bonds, generating (110) sheets. Very weak intra-sheet C-H⋯π inter-actions are also observed

RRI In-House meeting Photos - 2004 (Album - 3)

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CMB distortion from circumgalactic gas

Open AccessWe study the Sunyaev–Zel'dovich (SZ) distortion of the cosmic microwave background radiation from extensive circumgalactic gas (CGM) in massive galactic haloes. Recent observations have shown that galactic haloes contain a large amount of X-ray emitting gas at the virial temperature, as well as a significant amount of warm O vi absorbing gas. We consider the SZ distortion from the hot gas in those galactic haloes in which the gas cooling time is longer than the halo destruction time-scale. We show that the SZ distortion signal from the hot gas in these galactic haloes at redshifts z ≈ 1–8 can be significant at small angular scales (ℓ ∼ 104), and dominate over the signal from galaxy clusters. The estimated SZ signal for most massive galaxies (halo mass ≥1012.5 M⊙) is consistent with the marginal detection by Planck at these mass scales. We also consider the SZ effect from warm circumgalactic gas. The integrated Compton distortion from the warm O vi absorbing gas is estimated to be y ∼ 10−8, which could potentially be detected by experiments planned for the near future. Finally, we study the detectability of the SZ signal from circumgalactic gas in two types of surveys, a simple extension of the South Pole Telescope survey and a more futuristic cosmic-variance-limited survey. We find that these surveys can easily detect the kinetic Sunyaev–Zel'dovich signal from CGM. With the help of a Fisher matrix analysis, we find that it will be possible for these surveys to constrain the gas fraction in CGM, after marginalizing over cosmological parameters, to ≤33 per cent, in case of no redshift evolution of the gas fraction

RRI In-House meeting Photos - 2001 (Album - 1)

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Live cell plasma membranes do not exhibit a miscibility phase transition over a wide range of temperatures

Restricted Access.Lipid/cholesterol mixtures derived from cell membranes as well as their synthetic reconstitutions exhibit well-defined miscibility phase transitions and critical phenomena near physiological temperatures. This suggests that lipid/cholesterol-mediated phase separation plays a role in the organization of live cell membranes. However, macroscopic lipid-phase separation is not generally observed in cell membranes, and the degree to which properties of isolated lipid mixtures are preserved in the cell membrane remain unknown. A fundamental property of phase transitions is that the variation of tagged particle diffusion with temperature exhibits an abrupt change as the system passes through the transition, even when the two phases are distributed in a nanometer-scale emulsion. We support this using a variety of Monte Carlo and atomistic simulations on model lipid membrane systems. However, temperature-dependent fluorescence correlation spectroscopy of labeled lipids and membrane-anchored proteins in live cell membranes shows a consistently smooth increase in the diffusion coefficient as a function of temperature. We find no evidence of a discrete miscibility phase transition throughout a wide range of temperatures: 14–37 °C. This contrasts the behavior of giant plasma membrane vesicles (GPMVs) blebbed from the same cells, which do exhibit phase transitions and macroscopic phase separation. Fluorescence lifetime analysis of a DiI probe in both cases reveals a significant environmental difference between the live cell and the GPMV. Taken together, these data suggest the live cell membrane may avoid the miscibility phase transition inherent to its lipid constituents by actively regulating physical parameters, such as tension, in the membrane

Cubic to amorphous transformation of Se in silica with improved ultrafast optical nonlinearity

Restricted Access.We report a cubic to amorphous transformation of selenium in silica xerogel prepared through a sol–gel route. The crystalline nature of the nanoparticles examined from the SAED pattern and HRTEM revealed an α-cubic structure. The band gap energies calculated from the UV-vis absorption measurements showed a higher band gap of 3.5 eV for the cubic form compared to 2.4 eV for the amorphous form. The ultrafast optical nonlinearity of the samples was investigated by an open aperture Z-scan at 800 nm using 100 fs laser pulses. Although doping with cubic Se did not improve the optical limiting efficiency of silica xerogels, doping with amorphous Se results in substantial enhancement. Numerical analysis of the Z-scan data attributed the nonlinearity to two-photon and three-photon absorption and two-photon induced free carrier absorption. These intensity-dependent, instantaneous nonlinearities make amorphous Se doped silica xerogels attractive candidates for the design of ultrafast optical limiter devices

The spectral variability of the ghz-peaked spectrum radio source pks 1718-649 and a comparison of absorption models

Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)Using the new wideband capabilities of the ATCA, we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over 3 epochs spanning approximately 21 months, reveal variability both above the spectral peak at ˜3 GHz and below the peak. The combination of the low- and high-frequency variability cannot be easily explained using a single absorption mechanism, such as free-free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free-free optical depth on our line of sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of active galactic nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free-free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free-free absorption model characterized by a power-law distribution of free-free absorbing clouds. This agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free-free and synchrotron self-absorption) that are based on simple homogenous structure

The Murchison widefield array correlator

Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)The Murchison Widefield Array is a Square Kilometre Array Precursor. The telescope is located at the Murchison Radio-astronomy Observatory in Western Australia. The MWA consists of 4 096 dipoles arranged into 128 dual polarisation aperture arrays forming a connected element interferometer that cross-correlates signals from all 256 inputs. A hybrid approach to the correlation task is employed, with some processing stages being performed by bespoke hardware, based on Field Programmable Gate Arrays, and others by Graphics Processing Units housed in general purpose rack mounted servers. The correlation capability required is approximately 8 tera floating point operations per second. The MWA has commenced operations and the correlator is generating 8.3 TB day-1 of correlation products, that are subsequently transferred 700 km from the MRO to Perth (WA) in real-time for storage and offline processing. In this paper, we outline the correlator design, signal path, and processing elements and present the data format for the internal and external interface

All-sky signals from recombination to reionization with the SKA

Open Access, Proceedings of Advancing Astrophysics with the Square Kilometre Array (AASKA14).Giardini Naxos, Italy, from 9 -13 June, 2014Cosmic evolution in the hydrogen content of the Universe through recombination and up to the end of reionization is expected to be revealed as subtle spectral features in the uniform extragalactic cosmic radio background. The redshift evolution in the excitation temperature of the 21-cm spin flip transition of neutral hydrogen appears as redshifted emission and absorption against the cosmic microwave background. The precise signature of the spectral trace from cosmic dawn and the epoch of reionization are dependent on the spectral radiance, abundance and distribution of the first bound systems of stars and early galaxies, which govern the evolution in the spin-flip level populations. Redshifted 21 cm from these epochs when the spin temperature deviates from the temperature of the ambient relic cosmic microwave background results in an all-sky spectral structure in the 40-200 MHz range, almost wholly within the band of SKA-Low. Another spectral structure from gas evolution is redshifted recombination lines from epoch of recombination of hydrogen and helium; the weak all-sky spectral structure arising from this event is best detected at the upper end of the 350-3050 MHz band of SKA-mid. Total power spectra of SKA interferometer elements form the measurement set for these faint signals from recombination and reionization; the inter-element interferometer visibilities form a calibration set. The challenge is in precision polarimetric calibration of the element spectral response and solving for additives and unwanted confusing leakages of sky angular structure modes into spectral modes. Herein we discuss observing methods and design requirements that make possible these all-sky SKA measurements of the cosmic evolution of hydrogen

Tagged particle diffusion in one-dimensional gas with Hamiltonian dynamics-II

Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We study various temporal correlation functions of a tagged particle in one-dimensional systems of interacting point particles evolving with Hamiltonian dynamics. Initial conditions of the particles are chosen from the canonical thermal distribution. The correlation functions are studied in finite systems, and their forms examined at short and long times. Various one-dimensional systems are studied. Results of numerical simulations for the Fermi–Pasta–Ulam chain are qualitatively similar to results for the harmonic chain, and agree unexpectedly well with a simple description in terms of linearized equations for damped fluctuating sound waves. Simulation results for the alternate mass hard particle gas reveal that—in contradiction to our earlier results (Roy et al. in J Stat Phys 150(5):851–866, 2013) with smaller system sizes—the diffusion constant slowly converges to a constant value, in a manner consistent with mode coupling theories. Our simulations also show that the behaviour of the Lennard–Jones gas depends on its density. At low densities, it behaves like a hard-particle gas, and at high densities like an anharmonic chain. In all the systems studied, the tagged particle was found to show normal diffusion asymptotically, with convergence times depending on the system under study. Finite size effects show up at time scales larger than sound traversal times, their nature being system-specific