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Limits on low-frequency radio emission from southern exoplanets with the Murchison widefield array
No full textOpen AccessWe present the results of a survey for low-frequency radio emission from 17 known exoplanetary systems with the Murchison Widefield Array. This sample includes 13 systems that have not previously been targeted with radio observations. We detected no radio emission at 154 MHz, and put 3σ upper limits in the range 15.2–112.5 mJy on this emission. We also searched for circularly polarized emission and made no detections, obtaining 3σ upper limits in the range 3.4–49.9 mJy. These are comparable with the best low-frequency radio limits in the existing literature and translate to luminosity limits of between 1.2 × 1014 and 1.4 × 1017 W if the emission is assumed to be 100 per cent circularly polarized. These are the first results from a larger program to systematically search for exoplanetary emission with the MWA
The Murchison widefield array correlator
No full textRestricted 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
Size dependent nonlinear optical absorption in BaTiO3 nanoparticles
No full textRestricted Access.We present nonlinear optical absorption properties of BaTiO3 nanoparticles of different sizes prepared by the modified polymer precursor method. Structural properties of the samples were characterized using XRD and TEM, and optical properties by UV–visible light absorption. Nonlinear optical properties of the samples were measured by the single-beam open aperture Z-scan technique using 5 ns laser pulses at 532 nm. Results show that all samples exhibit a size dependent nonlinear optical response. Optical limiting efficiency increases with grain size and has a strong dependence on the structural phase of the particles. Nonlinear optical absorption is found to be enhanced when the particles undergo a transition from the cubic to the tetragonal phase
Tubular growth and bead formation in the lyotropic lamellar phase of a lipid.
No full textRestricted Access.We use fluorescence confocal polarised microscopy (FCPM) to study tubular growth upon hydration of dry DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) in water and water–glycerol mixtures. We have developed a model to relate the FCPM intensity profiles to the multilamellar structures of the tubules. Insertion of an additional patch inside a tubule produces a beaded structure, while a straight configuration is retained if the growth is on the outside. We use a simple model to suggest that reduction in overall curvature energy drives bead formation
Joint measurability and temporal steering
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)Quintino et al. [Phys. Rev. Lett. 113, 160402 (2014) [CrossRef] ] and Uola et al. [Phys. Rev. Lett. 113, 160403 (2014) [CrossRef] ] have recently established an intrinsic relation between nonjoint measurability and Einstein–Podolsky–Rosen steering. They showed that a set of measurements are incompatible (i.e., not jointly measurable) if and only if they can be used for the demonstration of steering. In this paper, we prove the temporal analog of this result, viz., a set of measurements are incompatible if and only if they exhibit temporal steering in a single quantum system
The cosmic dawn and epoch of reionization with the square kilometre array
No full textOpen Access, Proceedings of Advancing Astrophysics with the Square Kilometre Array (AASKA14).Giardini Naxos, Italy, from 9 -13 June, 2014Concerted effort is currently ongoing to open up the Epoch of Reionization (EoR) (15-6) for studies with IR and radio telescopes. Whereas IR detections have been made of sources (Lyman- emitters, quasars and drop-outs) in this redshift regime in relatively small fields of view, no direct detection of neutral hydrogen, via the redshifted 21-cm line, has yet been established. Such a direct detection is expected in the coming years, with ongoing surveys, and could open up the entire universe from 6-200 for astrophysical and cosmological studies, opening not only the EoR, but also its preceding Cosmic Dawn (30-15) and possibly even the later phases of the Dark Ages (200-30). All currently ongoing experiments attempt statistical detections of the 21-cm signal during the EoR, with limited signal-to-noise. Direct imaging, except maybe on the largest (degree) scales at lower redshifts, as well as higher redshifts will remain out of reach. The Square Kilometre Array(SKA) will revolutionize the field, allowing direct imaging of neutral hydrogen from scales of arc-minutes to degrees over most of the redshift range 6-28 with SKA1-LOW, and possibly even higher redshifts with the SKA2-LOW. In this SKA will be unique, and in parallel provide enormous potential of synergy with other upcoming facilities (e.g. JWST). In this chapter we summarize the physics of 21-cm emission, the different phases the universe is thought to go through, and the observables that the SKA can probe, referring where needed to detailed chapters in this volume (Abridged)
Foregrounds in wide-field redshifted 21 cm power spectra
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)Detection of 21 cm emission of H i from the epoch of reionization, at redshifts , is limited primarily by foreground emission. We investigate the signatures of wide-field measurements and an all-sky foreground model using the delay spectrum technique that maps the measurements to foreground object locations through signal delays between antenna pairs. We demonstrate interferometric measurements are inherently sensitive to all scales, including the largest angular scales, owing to the nature of wide-field measurements. These wide-field effects are generic to all observations but antenna shapes impact their amplitudes substantially. A dish-shaped antenna yields the most desirable features from a foreground contamination viewpoint, relative to a dipole or a phased array. Comparing data from recent Murchison Widefield Array observations, we demonstrate that the foreground signatures that have the largest impact on the H i signal arise from power received far away from the primary field of view. We identify diffuse emission near the horizon as a significant contributing factor, even on wide antenna spacings that usually represent structures on small scales. For signals entering through the primary field of view, compact emission dominates the foreground contamination. These two mechanisms imprint a characteristic pitchfork signature on the "foreground wedge" in Fourier delay space. Based on these results, we propose that selective down-weighting of data based on antenna spacing and time can mitigate foreground contamination substantially by a factor of ~100 with negligible loss of sensitivity
Serendipitous discovery of a dying giant radio galaxy associated with NGC 1534, using the Murchison Widefield array
No full textOpen AccessRecent observations with the Murchison Widefield Array at 185 MHz have serendipitously unveiled a heretofore unknown giant and relatively nearby (z = 0.0178) radio galaxy associated with NGC 1534. The diffuse emission presented here is the first indication that NGC 1534 is one of a rare class of objects (along with NGC 5128 and NGC 612) in which a galaxy with a prominent dust lane hosts radio emission on scales of ∼700 kpc. We present details of the radio emission along with a detailed comparison with other radio galaxies with discs. NGC 1534 is the lowest surface brightness radio galaxy known with an estimated scaled 1.4-GHz surface brightness of just 0.2 mJy arcmin−2. The radio lobes have one of the steepest spectral indices yet observed: α = −2.1 ± 0.1, and the core to lobe luminosity ratio is <0.1 per cent. We estimate the space density of this low brightness (dying) phase of radio galaxy evolution as 7 × 10−7 Mpc−3 and argue that normal AGN cannot spend more than 6 per cent of their lifetime in this phase if they all go through the same cycle
Cubic to amorphous transformation of Se in silica with improved ultrafast optical nonlinearity
No full textRestricted 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