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Modelling of the spectral energy distribution of Fornax A: leptonic and hadronic production of high-energy emission from the radio lobes
No full textOpen AccessWe present new low-frequency observations of the nearby radio galaxy Fornax A at 154 MHz with the Murchison Widefield Array, microwave flux-density measurements obtained from WMAP and Planck data, and γ-ray flux densities obtained from Fermi data. We also compile a comprehensive list of previously published images and flux-density measurements at radio, microwave and X-ray energies. A detailed analysis of the spectrum of Fornax A between 154 and 1510 MHz reveals that both radio lobes have a similar spatially averaged spectral index, and that there exists a steep-spectrum bridge of diffuse emission between the lobes. Taking the spectral index of both lobes to be the same, we model the spectral energy distribution of Fornax A across an energy range spanning 18 orders of magnitude, to investigate the origin of the X-ray and γ-ray emission. A standard leptonic model for the production of both the X-rays and γ-rays by inverse-Compton scattering does not fit the multiwavelength observations. Our results best support a scenario where the X-rays are produced by inverse-Compton scattering and the γ-rays are produced primarily by hadronic processes confined to the filamentary structures of the Fornax A lobes
Progression towards study of interactions with composite systems at ultra-cold temperature
No full textOpen Acces
Luminescent CdTe quantum dots incarcerated in a columnar matrix of discotic liquid crystals for optoelectronic applications
No full textRestricted Access.Here in we demonstrate, for the first time, the effects of highly luminescent alkylamine-capped semiconductor cadmium telluride quantum dot (CdTe QD) dispersion on the optical, electrical, and thermal properties and supramolecular order of a discotic liquid crystal (DLC). The insertion and properties of CdTe quantum dots in the columnar mesophase were studied by UV-vis spectroscopy, photoluminescence spectroscopy, polarized optical microscopy, differential scanning calorimetry, X-ray diffraction and DC conductivity. Results indicate uniform dispersion of CdTe QDs in a columnar matrix without disrupting the mesophase but enhancing the conductivity of the system significantly
Thermo-optic nonlinearity of the laser dye LDS 867 under low power CW laser excitation
No full textOpen Access, Book Chapter from International Conference on Materials Science and Technology (ICMST 2012), 10–14 June 2012, Kerala, IndiaThermally induced optical nonlinearity of the laser dye LDS 867 is studied in ethanol solution using the self phase modulation and closed aperture z-scan techniques, employing a continuous wave low power He-Ne laser beam for excitation. The nonlinear optical (NLO) coefficients are obtained by analyzing the z-scan curve on the basis of the thermal lens model. The dye exhibits a negative thermal nonlinearity which can be inferred from the occurrence of a pre-focal peak followed by a post-focal valley in the z-scan. The large nonlinear refractive index (n2) measured at the excitation wavelength of 633nm reveals that the material is NLO active even at low excitation powers of less than 1 mW. Results indicate that LDS 867 is a promising material for optical power limiting applications
Enhanced multi-phonon Raman scattering and nonlinear optical power limiting in ZnO:Au nanostructures
No full textRestricted Access.We report the synthesis of ZnO:Au hetero structures, their Raman scattering and optical power limiting efficiencies. ZnO:Au nanostructures with flower-like morphology were obtained by the self-assembly of ZnO nano entities using Au nanoparticles as seeds. Compared to ZnO nanostructures, ZnO:Au nanocomposites show enhanced UV emission and decrease in green emission. Increases in the intensities of Raman bands and multiphonon Raman bands of nano ZnO are observed substantially in the nanocomposites. Raman results further indicate that lattice vibrations of semiconductors are sensitive to the presence of metal nanoparticles. Open aperture Z-scan measurements carried out at 532 nm using 5 ns laser pulses reveal metal nanoparticle induced changes in the optical limiting properties of the nanocomposites
A digital receiver for the Murchison Widefield array
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)An FPGA-based digital-receiver has been developed for a low-frequency imaging radio interferometer, the Murchison Widefield Array (MWA). The MWA, located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia, consists of 128 dual-polarized aperture-array elements (tiles) operating between 80 and 300 MHz, with a total processed bandwidth of 30.72 MHz for each polarization. Radio-frequency signals from the tiles are amplified and band limited using analog signal conditioning units; sampled and channelized by digital-receivers. The signals from eight tiles are processed by a single digital-receiver, thus requiring 16 digital-receivers for the MWA. The main function of the digital-receivers is to digitize the broad-band signals from each tile, channelize them to form the sky-band, and transport it through optical fibers to a centrally located correlator for further processing. The digital-receiver firmware also implements functions to measure the signal power, perform power equalization across the band, detect interference-like events, and invoke diagnostic modes. The digital-receiver is controlled by high-level programs running on a single-board-computer. This paper presents the digital-receiver design, implementation, current status, and plans for future enhancements
The spectral variability of the ghz-peaked spectrum radio source pks 1718-649 and a comparison of absorption models
No full textRestricted 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
Structural investigation of resorcinol based symmetrical banana mesogens by XRD, NMR and polarization measurements
No full textRestricted Access.Synthesis and structural characterization of two novel symmetrical banana mesogens built from resorcinol with seven phenyl rings linked by ester and imine with a terminal dodecyl/dodecyloxy chain has been carried out. Density functional theory (DFT) has been employed for obtaining the geometry optimized structures, the dipole moments and 13C NMR chemical shifts. The HOPM and DSC studies revealed enantiotropic B2 and B7 phases for the dodecyl and dodecyloxy homologs respectively. The powder X-ray studies of both the mesogens indicate the presence of layer ordering. The polarization measurements reveal an anti-ferroelectric switching for the B2 phase of the dodecyl homolog whose structure has been identified as SmCSPA. The B7 phase of the dodecyloxy homolog was found to be non-switchable. High resolution 13C NMR study of the dodecyl homolog in its mesophase has been carried out. 13C–1H dipolar couplings obtained from the 2-dimensional separated local field spectroscopy experiment were used to obtain the orientational order parameters of the different segments of the mesogen. Very large 13C–1H dipolar couplings observed for the carbons of the central phenyl ring (9.7–12.3 kHz) in comparison to the dipolar couplings of those of the side arm phenyl rings (less than 3 kHz) are a direct consequence of the ordering in the banana phase and the shape of the molecule. From the ratio of the local order parameter values, the bent-angle of the mesogen could be determined in a straight forward manner to be 120.5
Self-assembly of thiolated graphene oxide onto a gold surface and in the supramolecular order of discotic liquid crystals
No full textRestricted Access.Graphene oxide can be covalently functionalized with thiol molecules to produce thiolated graphene oxide which shows interesting behaviour of self-assembly on gold surfaces and in the supramolecular structures of discotic liquid crystals. The formation of a self-assembled monolayer on the gold surface was confirmed by electrochemical, XPS, SEM, and grazing angle IR studies. Supramolecular nanocomposites of anthraquinone based discotic liquid crystals with thiolated graphene oxide were characterized using SAXS, POM, DSC and Cryo-SEM. Such self-assembling supramolecular nanocomposites open up new and interesting possibilities for fundamental studies as well as for optoelectronic applications
