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Non-linear multipole interactions and gravitational-wave octupole modes for inspiralling compact binaries to third-and-a-half post-Newtonian order
No full textOpen Access - IOP selectThis paper is motivated by the need to improve the post-Newtonian (PN) amplitude accuracy of waveforms for gravitational waves generated by inspiralling compact binaries, both for use in data analysis and in the comparison between post-Newtonian approximations and numerical relativity computations. It presents (i) the non-linear couplings between multipole moments of general post-Newtonian matter sources up to order 3.5PN, including all contributions from tails, tails-of-tails and the non-linear memory effect; and (ii) the source mass-type octupole moment of (non-spinning) compact binaries up to order 3PN, which permits completion of the expressions of the octupole modes and of the gravitational waveform to order 3.5PN. On this occasion we reconfirm by means of independent calculations our earlier results concerning the source mass-type quadrupole moment to order 3PN. Related discussions on factorized resummed waveforms and the occurence of logarithmic contributions to high order are also included
Productivity of Indian Telescopes: Impact analysis through scientometric methods
No full textOpen Access, Proceedings of Library and Information Services in Astronomy (LISA) VII
on Open Science: At the Frontiers of Librarianship, held INAF, Astronomical Observatory of Capodimonte, Naples, Italy. From 17th to 20th June 2014, Edited by A. Holl et alThis paper aims to study the productivity of modern Indian telescopes that were installed after India attained independence from colonial rule. (The study also includes one telescope built under colonial rule that is still in operation today.) Productivity is measured by the number of papers published from the fourteen telescopes included in the study. Researchers make use of the astronomical data generated by these telescopes and write research papers, which are then used by other researchers and cited in subsequent papers. This study quantifies the impact that the telescopes have on scholarship and analyzes the data using scientometric indicators
Possible influence of layer deformation and Chiral segregation on dielectric modes in the dark conglomerate liquid crystal
No full textRestricted Access.Dielectric spectroscopy is used to investigate the structure, molecular dynamics, and relaxation phenomena in electric-field-induced switchable dark conglomerate (DC) phases in a bent-core liquid crystal. The DC phases are obtained by applying a high-frequency ac electric field in the B1rev phase or by cooling under a dc or an ac field from the isotropic phase. Although the DC phases exhibit good electro-optic switching properties, the dielectric parameters are different from those observed in typical lamellar SmCP phases and similar to those obtained in a non-switchable DC phase. We therefore propose that the dielectric response and reduced intensity of the relaxation modes may be a general feature in DC phases and may owe its origin to the deformed layer structure in which certain molecular motions are impeded. Further, we find that in the field-induced DC phases derived from the isotropic phase, the dielectric modes are affected by chiral segregation promoted by the applied field
The chemistry of bent-core molecules forming nematic liquid crystals
No full textRestricted Access.Analogous to rod-shaped and disc-shaped molecules, bent-core molecules also exhibit nematic phases, in addition to other higher order mesophases. Presently, about 600 bent-core or banana-shaped molecules are known to exhibit nematic phases. In this article, the chemistry and mesomorphic properties of all these materials are summarized
How late can the dark matter form in our universe?
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We put constraints on the epoch of dark matter formation for a class of non-WIMP (Weakly Interacting Massive Particle) dark matter candidates. These models allow a fraction of Cold Dark Matter (CDM) to be formed between the epoch of Big Bang Nucleosynthesis (BBN) and the matter radiation equality. We show that for such models the matter power spectra might get strong suppression even on scales that could be probed by linear perturbation theory at low redshifts. Unlike the case of Warm Dark Matter (WDM), where the mass of the dark matter particle controls the suppression scale, in Late Forming Dark Matter (LFDM) scenario, it is the redshift of the dark matter formation which determines the form of the matter power spectra. We use the Sloan Digital Sky Survey (SDSS) galaxy clustering data and the linear matter power spectrum reconstructed from the Lyman-α data to find the latest epoch of the dark matter formation in our universe. If all the observed dark matter is late forming, we find lower bounds on the redshift of dark matter formation zf > 1.08 × 105 at 99.73 % C.L from the SDSS data and zf > 9 × 105, at the same C.L, from the Lyman-α data. If only a fraction of the dark matter is late forming then we find tentative evidence of the presence of LFDM from the Lyman-α data. Upcoming data from SDSS-III/BOSS (Baryon Oscillation Spectroscopic Survey) will allow us to explore this issue in more detail
All-sky signals from recombination to reionization with the SKA
No full textOpen 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
Confirmation of wide-field signatures in redshifted 21 cm power spectra
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We confirm our recent prediction of the “pitchfork” foreground signature in power spectra of high-redshift 21 cm measurements where the interferometer is sensitive to large-scale structure on all baselines. This is due to the inherent response of a wide-field instrument and is characterized by enhanced power from foreground emission in Fourier modes adjacent to those considered to be the most sensitive to the cosmological H i signal. In our recent paper, many signatures from the simulation that predicted this feature were validated against Murchison Widefield Array (MWA) data, but this key pitchfork signature was close to the noise level. In this paper, we improve the data sensitivity through the coherent averaging of 12 independent snapshots with identical instrument settings and provide the first confirmation of the prediction with a signal-to-noise ratio \gt 10. This wide-field effect can be mitigated by careful antenna designs that suppress sensitivity near the horizon. Simple models for antenna apertures that have been proposed for future instruments such as the Hydrogen Epoch of Reionization Array and the Square Kilometre Array indicate they should suppress foreground leakage from the pitchfork by ∼40 dB relative to the MWA and significantly increase the likelihood of cosmological signal detection in these critical Fourier modes in the three-dimensional power spectrum
Numerical studies of dynamo action in a turbulent shear flow. I.
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We perform numerical experiments to study the shear dynamo problem where we look for the growth of a large-scale magnetic field due to non-helical stirring at small scales in a background linear shear flow in previously unexplored parameter regimes. We demonstrate the large-scale dynamo action in the limit where the fluid Reynolds number () is below unity while the magnetic Reynolds number () is above unity; the exponential growth rate scales linearly with shear, which is consistent with earlier numerical works. The limit of low is particularly interesting, as seeing the dynamo action in this limit would provide enough motivation for further theoretical investigations, which may focus attention on this analytically more tractable limit of compared to the more formidable limit of . We also perform simulations in the regimes where (i) both (, ) < 1, and (ii) and , and compute all of the components of the turbulent transport coefficients ( and ) using the test-field method. A reasonably good agreement is observed between our results and the results of earlier analytical works in similar parameter regimes
A systematic study of coding performance in a MIMO-STBC-OFDM link
No full textRestricted Access.MIMO–STBC–OFDM systems mitigate inter symbol interference and frequency-selective fading caused by multi-path propagation in modern high-data-rate wireless communication. Error correction codes use redundancy to correct errors that still occur. No systematic analysis of their performance using a common link configuration is found in the literature. We fill this gap in this simulation study using a common MIMO–STBC–OFDM communication link, transferring data at a symbol rate of 100 Mbps. We find that (a) for all code rates, low-density parity-check code performs best with BPSK and to an extent with QPSK and (b) ½-rate convolutional code works well for all symbol mapping schemes
